A car subwoofer need lots of rms power to produce hard hitting bass, if you dont have an amplifier or you dont want to buy it, then you can wire your car subwoofer to the head unit, but you will have to sacrifice a rear or front speaker in your car. Wiring a subwoofer to the head unit will power your car subwoofer without any problems, but your subwoofer will not play bass really loud because most aftermarket and stock head units produce like 25 watts rms on each channel. Hooking you sub to your deck will make the sub not play exact bass frequencies below 100Hz, as the head unit will send mids and highs frequencies to the subwoofer.
To wire your car subwoofer to your head unit, connect the speaker wire to the car subwoofer and remove a rear speaker connection thats connected with the speaker connections on the headunit plug. To get access to the headunit plug, you will need to remove the head unit from the cars dashboard using din tools. Once you have disconnected the connections that provide sound to a rear speaker in your car, run your subwoofer cable through your cars interior and connect the subwoofer cable to the rear speaker connection on the car head unit wiring plug. When you have made the connections to the subwoofer, use insulation tape to cover up any exposed braid of the speaker wires; this is to prevent crackling to occur.
Once you have done that your subwoofer will be powered with the built in amplifier inside the head unit. To optimize the bass response of the subwoofer, turn down the treble to the lowest level and turn up the bass to the maximum level. To cancel out the mids and high frequencies that are sent to the car subwoofer altogether, install a crossover over with a low pass filter in line with the subwoofer and head unit.
To make you subwoofer play bass loud and hit harder, you can disconnect another rear or front speaker and bridge the channels on the head unit. This will allow your head unit to produce more power for you sub. Head unit bridging can actually damage the amplifier inside it because the internal amplifier inside it can overheat. So i suggest you place a cooling fan on the rear of the headunit to blow cool air inside it.
