I just did this same thing. I removed the spare tire from the tire well and built a custom subwoofer enclosure for an 8" JL Audio sub but any passive sub in an enclosure would work. I looked into many options and decided to just run the rear speakers off the head unit amplifier (they mostly provide non-directional fill anyway). I installed bass blocking capacitors on those rear speakers to filter out the low frequencies they cannot handle since they are no longer using the cross-overs in the amp. Most 4 channel amps supports a 2 x stereo (front speakers) + 1 x bridged Mono mode (Sub). You can then use the cross-overs built into the amp to filter out the lows as appropriate for the front speakers and the highs from going to the sub. Set the cross overs to the speaker manufacturer's suggested settings and then tune from there. Use the fade capability on the head unit to get your desired front rear bias. Then us the gain settings on the amp to get your sub to the desired level. Also with the bass blockers and cross overs setup correctly you can use the bass control on the head unit to tweak the amount of subwoofer bass depending on what you are listening to. Sounds great and you don't have to spend much more money at all. Just need a passive subwoofer in an enclosure and some cheap bass-blockers.
