I have a car that came with a proprietary audio system. The head unit sent a single L/R line level signal to the custom amp, which then split the signal to various speakers: front mids, front tweeters, rear mids, front center, and a single sub. The amp appears dead and replacements run in the $2,500+ range. I am just trying to get the system (mostly) working again on the cheap with a spare head unit I had. I am just using the head unit's built in amp to power the front mids/tweeters via a set of passive crossovers, but I am curious if I can get the sub working the same way. I am thinking to use another set of crossovers, three-way this time, to split the rear signal from the head unit into 3 sets of outputs - 1) woofer, 2) mid, and 3) highs. The mid outputs will be wired into the rear mid speakers and the tweeter outputs will be unused. My question is about the woofer outputs. The sub is a single voice channel. I've read enough to know that I should not simply wire the L/R + and L/R- to the sub, but what about just using a single +/- from the left channel (for example) off the crossover while leaving nothing connected to same right channel. Any technical issues with this setup? Is there a way to wire in the L/R+ and L/R- together by adding in 4-ohm resistors or otherwise make it into a mono circuit? If not I can always leave the sub un-used and it still sounds okay.