I remember seeing a fancy antique car in a museum from the twentieth century teens, I think. I don’t remember the make.
It had two little vases on each side in the back seat, located about where the shoulder strap dispenser would be mounted in a modern car. Two little vases to put flowers in. Matching bouquets, no doubt. A museum attendant had to tell me what the little vases were for, flower vases for cars was that foreign a concept to me.
Not according to Wikipedia:
mmm
I remember my dad installing seatbelts before the mfrs. were forced to put them in. They weren’t the metal-to-metal buckles like for the past, what, forty years, but rather you’d slide a stiff nylon belt through a spring-loaded latch with gripper teeth. I was dubious even as a child whether it would hold tight in an accident. He was very happy when they started putting anchor points in so you didn’t have to figure out where to drill through the floor pan without hitting something important.
I had one of those cassette to 3.5mm aux converters in several cars until well into the 2000s - initially for using a Discman and later for running MP3 players.
Similarly, until a few years ago I was still using a bluetooth FM transmitter that would connect your phone to the car’s FM radio. Very handy since it’s illegal to use a mobile phone while driving without a handsfree kit in Queensland.
AirGuide was one manufacturer. These were a common accessory. Since a car is made largely of steel/iron, they need to be calibrated when installing, this involves parking the car pointing N/S and E/W performing a compass swing and boxing the compass in set screw adjustment.
I suppose GPS and smartphone apps and such has obviated the need, but these were super useful. Grandpa wasn’t as dumb as he looked. When travelling in remote areas especially, poking around exploring lots of gravel roads, there are times when it’s not really critical what particular road to take, just that you’re going the right direction, i.e. we’ll run into the N/S hardtop sooner or later, but want to make sure we’re generally going west, say. I had one that I picked up in a duty free shop of all places and they were really handy. If I can find a good one for a good price I’m going to buy it, because analog dials and equipment are just cool. The better models even had lighting.
Here’s one you probably forgot about:
When door locks were activated by pressing down those little knobs/buttons that popped up, it was common to replace them with longer ones, because many stock versions sat too flush to the door to grab them easily to pull them up to unlock the door (safety fail 101).
Also, while hub caps are still a thing, you just don’t see wheel trim rings much anymore.
I had a 1953 Pontiac that came with this factory installed. They had to, as the car also came with a BIG sun visor mounted above the windshield on the outside of the car, so that any traffic light hanging over the center of the intersection would be completely hidden from the driver.
This car had an interesting idiosyncrasy - it had a big straight eight engine (low power, but a gas hog) that had the habit of eating the #7 piston. Luckily, in those days you could get the engine repaired for around a hundred bucks (IIRC).
The longer ones I remember were not only longer, but straight as well, rather than being flared at the top. Made it a bit more difficult to unlock the door from the outside with a strategically-bent coat hanger.
I’m a bit surprised no one mentioned the reverb. Motorola made the original Vibrasonic, other makes used other names. They usually mounted under the package shelf and connected to the rear speaker. Consisted of a tensioned small spring, the signal went into one end of it and bounced back and forth for a while, coming out the other end. Some music sounded really good that way. Whiter Shade of Pale was amazing! If you drove over a rr track it sounded like somebody dropped a piano in the trunk. We used to turn on baseball games and they sounded like 20,000 fans all shouting into barrels.
I also went thru the usual horns, driving and fog lights, outside mirrors on the cars that didn’t have them already. Signal-Stat made a kit that you could add 4-way flashers to cars, As that feature was not standard until about 1968. It had about 20 wires coming out of the thing, it mounted under the dash and had a big red knob you pulled out to activate it. It came with about a 50 page manual which covered all kinds of cars, domestic and imports, you only used certain of the wires for your specific vehicle. It took me about a week of evenings to put into a 1965 Buick Skylark. The 1964 Corvair only took a couple of nights.
Inlet manifold) Vacumn guage, turn-signal audible buzzer, windshield washers, and, in the words of the imortal song “twin overhead foxtails”
Aftermarket power windows, anybody? They’re still being made and sold
I had a 1988 Chevy S10 that I bought in 1990. The original owner had gone to AutoZone and bought about every aftermarket accessory they sold for it. It had a light bar with fog lights, fog lights on the front bumper, pink neon tubes on the underside (the truck was grey), an add on tachometer, an aftermarket stereo and speakers, an aftermarket sunroof (NEVER buy a vehicle with one of these), a bed mat, Yosemite Sam mudflaps, a Bugs Bunny shift knob, Tweety Bird seat covers, a pink steering wheel cover, wind deflectors over the windows and a horn that played the Loony Toons theme. It was just a teeny bit conspicuous. 
ETA: It also had aftermarket pink pinstriping with the Chevy heartbeat squiggle.
For my first car, I had an aftermarket rear window defroster that was nothing more than a 12-volt space heater with a fan. Noisy but generally effective by the time I got to my destination.
My first new car was a 1991 Honda Civic DX hatchback. It didn’t come with a radio, clock or even a passenger side door mirror. No power steering either, although it wasn’t a big deal considering how light the car was. I went the aftermarket stereo route also, but had to buy the mirror, clock and armrest from the dealer (for a ridiculous price).
I stlll remember the anti frost shields that Dad would install on the side windows to prevent frost on the inside windows. They generally worked about a month before the plastic cracked from the cold or from an incomplete seal. Also, in car heaters that you lugged in with your block heater.
Also,
Spot lights you could move around a la Cop cars.
Boogie vans , generally.
Dual car antennas
Hood pins
Add-on tachometer
Chrome headlight half covers
The worst accessories from the late 80’s were double windshield wipers, these are all the rage in 1986. The other stupid one we fake cell phone antennas.
Yep
Sounded like a good idea for a VW bug with tiny wipers to start with, except that the blades were doubly crap, and even if you replaced them with good blades, the wiper arms only are set up with the spring strength to properly push down a single blade.
OK no problem, put a stronger spring in, well now the wiper motor cant move the things correctly.
Worked out much better to just put bosh refills in the stock blade holders, clean the windshield really good with some 000 ultra fine steel wool to removed the crap that oxidizes to it, and then wax the windshield glass, cheaper too.
I so wanted those double wipers to work! The set I got also had the washer fluid squirter option built in, so the fluid would squirt on the windshield in between the blades. Didn’t work well at all.
Here is a tip for any U.S. Toyota Yaris owners. If you have a Yaris without cruise control, go to yarisworld.com and do a search for DIY cruise control. There are excellent instructions for activating the cruise control built into all of them- they just leave the switches off on cars without the option, and the site shows you how to add them by either buying the stock parts or by using stuff available at Radio Shack (or Fry’s nowadays, I guess).
Same with the MG Midget. I remember ours had a set of sails to press the tiny blades onto the screen harder with the windspeed.
One of my high school friends had a “color organ” installed in his dashboard; this was a small translucent screen with colored lights behind it, and the lights would respond to frequencies in the music playing on the car stereo.
It was very cool, but very distracting. It would probably be illegal today, if not then (this was during the early 1970s).