Now, I’m pretty cheap, so I’m looking to save money. I want a subwoofer in my car, but I don’t want to pay for it. I have an extra sub in my house, and was wondering if there was any way to safely install it in my car. I’ve been searching all over the internet to no avail… so I came to you guys.
Most source units have preamp outputs for subwoofers so all you’ll probably need is an amplifier to drive the sub (assuming it is a passive sub, ie it has no amp of its own).
But you have to tell us exactly what brand/model is the subwoofer and the source unit, else we are shooting in the dark.
You’re probably going to run into impedance issues; IIRC, house speakers have about twice the impedance of car speakers.
Home subwoofers usually have their own amp within the enclosure and run off of AC power. This poses two problems for you, supplying AC power (you will need to purchase an inverter of sufficient capacity) and getting the signal to the sub. This is a very poor and jury rigged situation. You may even find that you are having battery/alternator problems. If you are doing a clean installation, you will be able to save money on the head unit, they don’t usually differ in power, across the manufacturer’s line, they only add more eq features and bling.
Perhaps you could use the speaker by bypassing the existing amp and plumbing that to a car stereo amp. Car stereo speakers tend to be 4 ohm, home speakers can range from 4 ohm to 8 ohm, though the connection on the back of the sub may be just a line level connection to the amp.
One thing a lot of people don’t plan for is the additional current draw. In a typical setup there is an external amp for the subwoofer (and often the speakers) that is connected with a thick cable to the battery. There may even be a capacitor in the line, to smooth out the current draw to the battery. This won’t change the overall power consumption, just keep your headlights from dimming every time there is a thump on the woofer. Ideally you also want a high output alternator, that can keep up with the additional current draw.
What I am trying to say is that you can run into trouble (such as your car dying from a dead battery while you are jamming your tunes on the highway) if you try to do things half-assed. Installing a high-end car stereo system is something that if you do not know what you are doing, you can run into trouble that goes beyond the rig sounding bad or not functioning.
And there is loads of info on the net about how to do it if you want to try it yourself, but you should read up first, or take it to a pro.
Given a day or two to subconsciously cognate on your situation, I have some advice. It would help to know what type of vehicle are you putting it in, how long do you want to keep it, what kind of listening do you do, etc?
Assuming it is in good shape, why don’t you put the existing sub on eBay or Craigslist and apply the proceeds to a sub that is optimized for use in a vehicle. You may be able to find an easy solution with a contained amp in an appropriate enclosure, for around $100.
Anything more than that would probably make an alternator upgrade advisable. I have run my battery down jamming tunes using just my 4x25w head unit. Engine died. Trying the starter produced the dreaded click. I was lucky to have a street legal place close enough to coast into to park the van overnight, and I was two minutes from home so it was easy to call a neighbor to come get me. The next day, after the battery had recovered, it turned right over. I got lucky. I would not want that to happen miles from home on the freeway. The cost of a tow alone could buy you a beefed up alternator and a nice amp for your sub.
I drive a full sized Dodge Van with big honking alternator, battery, etc., and it happened to me. You better believe I put a high output alternator in my thing before I started fooling with amps and subwoofers. Once you start adding powerful subs and amps you are asking for trouble.
Capacitors don’t help. Extra batteries don’t help, although both may be desirable, but If your system is consistently drawing more than the alternator is putting out, your battery will eventually be picking up the slack and at some point will not have enough to spark your spark plugs, and then it is AAA time.
FYI, My system is a Pioneer, and I have never had a problem with any of their home or mobile equipment. Aside from the fact that after two years, I still can’t figure out all the things the head unit can do. 
I got the one-step-up from base head unit, I think it was around $129. But it drives all my other high end stuff fine. Another nice thing about buying the maybe one-or-two steps up from the base model, is that it doesn’t advertise to the world that you have a killer system. Spend your money on stuff that the Seab’s of the world can’t see, like speakers, amps, etc.
Another question is, can you get at the rear of the head unit to connect the sub? Does the head unit even have an output for the sub? Can you take apart the dash and gain access? Some of these newfangled dashboards are like a puzzle to get apart. One reason I love my good 'ol Dodge Ram Van, is all you have to do to take the durn thing off is look for a bunch of ugly Phillip’s Head screws and take them off, one by one. Same with all other pertinent interior trim. Newer cars, fuggediboutit. Take it to Best Buy and try to get the installation bundled with the system.
But don’t be like a lot of clowns out there. If you are consuming serious wattage, and I think that 4x25 plus a small sub would wipe you out, you must upgrade the alternator. A lot of people don’t do that and they may have learned by experience similar to what I related above, that yes, they can annoy people at stoplights, but if they try to “enjoy” their music for a long period of time, they will be waiting for their battery to recover from the incident.
The neat thing about improving your car audio, is that the components to put together a very nice system aren’t expensive, individually, and can be added one by one. It is also a great realm for the tinkerer.
I will try to dig out a link to the supplier of my alternator, it was less than $200, and that is where I would actually start, if you want to go bigger.
Let us know how you decide and how your project works out. Happy listening!
—No, I am not the guy next to you thumping the screws out of their fittings at the stoplight, so everybody relax. I don’t usually even listen to rap. Some of us, (<1%) just like to listen to a clean system without a lot of bass boost in the eq., and listen to a wide variety of music. 