Powering car amp and subs off spare car battery?

absolutely. i actually have a set up similar to that in my room. but you need to put something down under the battery if its in the truck, to catch the battery acid. just hook the power cable up to the positive and the ground wire to the negative and a really long wire from the remote slot to the head unit. the other problem is the power it will take to push it. the speakers will probably start cutting out on hard hits, because that would be running the battery off of the CCA (Cold Cranking Amps). the alternator is what would actually be powering the subs most of the time. it would be best to just wire it directly to the battery already in the car.