To get back to the OP, you could just put a low-amp space heater on the passenger seat and plug it in with a 12V adapter. That seems a bit Simpsonesque, though, somehow.
I dug through that site to find out that that thing is about 480W. Too much for your car cigarette lighter, which is probably on a 10 or 15 amp circuit.
I second the vote for “seat warmers are better as a quick way to warm up before the warm air kicks in”.
Seat heaters are hard to retrofit into an existing car, but here are some simple add-on:
http://www.walmart.com/ip/Wagan-Heated-Seat-Cushion/10928415
There are also a few companies that make 12V heated clothing for motorcyclists. It works pretty well. A heated vest is a big help. It is also possible to sew heating wire into garments. I won’t offer specifics, because if you screw up it is easy to burn yourself or even set yourself on fire. It is all about achieving low power density. I will note that you want the heating elements to operate at fairly low temperature, so nichrome or other exotic alloys are not required. Copper works, but tends to fail from mechanical fatigue in clothing, and fairly fine wire is required due to copper’s low resistivity. You can get stainless fishing leader material in bulk that works well. It is high resistance, so you don’t need the hair thin strand that you need with copper.
Yes I ride motorcycles. My truck is a slow-to-heat-up diesel.
I don’t consider 48 volts to be a shock hazard, unless your hands are wet, and even then, it isn’t like 120 volt household power (speaking from experience). Also, they can use 48 volts for the main electrical system (note that electric cars usually use hundreds of volts for the motor, no reason the heater can’t run directly off the main battery either, reducing current to a few amps for a 1 kW heater) and convert that to 12 volts for lower power accessories and the cigarette lighter plug.
Anecdote time: I had a buddy with a cheap-ass Kia (2005 Kia Rio, IIRC) which had almost instant-on heating. I’m not sure how it worked.
I do remember I was pissed that my substantially more expensive car bought the same year did not, though.
Perhaps it just had a tiny engine. A small engine will be operating closer to it’s rated power, so making more heat for it’s size, and due to small size will heat up much faster.
Also it probably had a transverse mounted engine, which means it had an electric radiator fan instead of belt driven fan. The belt driven fans blow cold air over the engine whether they need it or not. The electric fans are controlled by a thermostat, so only run as needed for cooling…perhaps not at all when it is really cold out.
Finally, it is really common for thermostats (valve in cooling system) to fail open, causing very slow warm-up and poor heater performance, as well as poor fuel efficiency, increased wear, and high emissions. These are cheap and usually not to hard to replace, but it is probably one of the more neglected maintenance items due to “out of sight, out of mind” effect, and the failure can mostly be lived with.
Sometimes I feel like I’m 100. 
I’m 55 and lived through the 60s. And the first car I learned to drive stick on was a '66 Beetle.
I’m only in my 30’s, but have had a few as daily drivers. If you lived in a generally rust-free area, they were still perfectly practical for transportation well into this century. Unlike most classic cars, the parts kept getting cheaper and cheaper as they kept churning out new “old” bugs in Latin America. For a DIY cheapskate they were the perfect car even 30-40 years after the things were built.
OK, I give, I’ll join the hijack:
I still see lots of air-cooled beetles on the road. I live in the desert southwest where low humidity and limited use of road salt keep rust to a minimum. I even know one guy that drives a “Thing”.
Ferdi Porsche designed the VW for longevity via easy serviceability. Bob Hoover, (not the pilot) now departed, used to write about the “forever car”. You really could/can keep one running indefinitely without special tools or knowledge.
Which is the opposite of the current company.
In the cosmic scheme of things air cooled VW were pretty terrible cars both then and now.
Your basic bug was sate of the art 1935 or so. It was no more reliable than any other car built in the at the time and had some huge issues:
3 quart sump, which would not pick up the last quart and a half. Which wouldn’t be so bad except they leaked. A lot.
The heater was a joke unless you were moving down the road, but that was OK because the carb was located right over the generator full of sparks so when the fuel inlet vibrates out of carb you could warm you hands on the resulting engine fire.
What lead to VWs success was three things
Excellent parts availability. Blow your engine up? No problem the dealer stocked all the parts and you would be back on the road that day.
Superb advertising. VWs print ads are iconic which let to
A cult status among owners.
Lose the parts availability or great ads and VW would have been another used to be sold brands.
I don’t agree with them not being reliable. If you were not a wrench turner then they could be unreliable. I spen many years driving a lot of different VWs. I did not have any more road side break downs than other cars that were not new. And I am a wrench turner so I knew if it was ok to take a long trip from home or not. And if it was not then I serviced or repaired what ever was doubtful and went. And they were easy to fix. in tge 50"s and 60"s VW ads claimed that they could build a complete car out of the parts department.
One of the few cars I know of where one person can physically pull the engine out and work on it.
And in 20 minutes.
They were definitely not as robust as the overbuilt cars Detroit put out in the 50’s and 60’s, but they were far, far better cars than the other small imports that made a go of it in the states during that period. And of course Detroit didn’t sell anything remotely comparable-- their idea of a “small” car was still a big slab of steel like a Falcon or a Skylark. Until the Japanese really came onto the scene in the 70’s, the VW was absolutely the best car you could get in its size and price range.
I worked in a foreign car repair shop in the early 1970s. We had several guys working on VWs I did everything else that came in the door.
The standard joke in the shop was my customers got a tune up every 10K miles, while the VW owners got a valve job or a whole new engine every 10k. No kidding we had customers with 40K on their bug and had already had three valve jobs and a complete engine.
A Datsun 510 or a Toyota Corona with a 3RC engine kicked the shit out of a VW in reliability.
so, they were unreliable.
PROTIP: being able to (constantly) fix your car doesn’t make your car reliable.
this isn’t really a ringing endorsement of anything. The Beetle was a piece of shit; the only reason people cared about it is because there wasn’t anything else like it.
I too drove a couple of Bugs over the years- a '66 and a '71. Neither car was more unreliable than any other car I ever owned with a points ignition, with unreliable being defined as the car not being able to get me where I wanted to go. However, the only parts of the '66 that weren’t rusted away were the visible, exterior body parts, which meant the heater, about which this conversation about VWs started, was totally nonfunctional. I used to have to use an ice scraper on the inside of the windshield in winter to scrape my frozen breath away so I could see. On the '71, the sheet metal was OK, but all the soft parts of the heat system had failed, leaving me without heat or defrosters. This was in south Georgia, however, so it didn’t really matter. I had these cars in the early to mid-80s and the best highway mpg I ever got from either of them was about 29-30 at 55-60 mph.
Really, when I sold the '66 for $125 and bought a beat-up, rusty '73 Vega wagon for $300, it was like dying and going to automotive heaven. The heater and defrosters worked fine in the Vega and the gas mileage was just as good. A car’s pretty bad when a Vega wagon upstages it.
If you skipped the tune ups/services on a VW you would be replacing the engine. And services were every 3000 miles. The early 1600 engines had valve guide problems that could cause the valve to break. The trick was to check the valve clearance with every service, and if the clearance was low each sevice then the valve guides would need to be replaced. As I said earlier if you drove one and were a wrench you were OK. But if you had to pay someone to keep it running lthen it could get expensive. The 36Hp engines were good for over 100,000 miles.