How to connect a subwoofer to a car without a separate amplifier?

Most modern car stereos have an RCA subwoofer output, you just plug the sub straight into that and you're good to go. If your head unit doesn't have that output, you can use a high-level to low-level impedance adapter, but honestly it doesn't work as well. The thing is, the powered sub handles its own volume, so there's nothing complicated to install, just the cables and the 12V power from the battery.

If your car stereo is really old and doesn't have a subwoofer output, you can also hook it up directly to the speaker wires with a cheap line converter (RCA), but you gotta check that your subwoofer can handle that because some of them absolutely need a separate power supply.

Just gotta watch out for one thing the others didn't really mention: before you plug anything in, check your car stereo's output power because some active subs really demand a lot of juice and if you've got a cheap entry-level head unit, you're gonna end up with crappy sound or worse, you'll blow stuff. After that yeah, if you've got a dedicated RCA sub output it's definitely the easiest route, but if you gotta mess around with a converter, make sure your sub's got a decent frequency response too or you're just gonna get boomy bass without any real quality!

Wait, one mistake you absolutely need to avoid: never connect the subwoofer directly to the speaker wires without going through a converter, or you'll fry your gear. What people often forget is that even with a sub RCA output on the car stereo, you need to gradually adjust the subwoofer level by testing it first at low volume, because lots of people just crank the power all the way up right away and then it sounds terrible and the battery drains fast 😅