good, stereo->i can help with that at least. What you want to do is wire the batt's in parrallel, so run a large wire from your main battery posistive to your second batt positive terminal (use at least 4awg wire, recomend 0awg though) and you can wire the amp from the second batt pos. For the ground, just bolt a short wire of the same size as the power wire to the chassie and make sure the metal is bare and clean (no paint or rust). Your amp can share the same ground as the batt, it wont hurt anything.
This setup will double your useable amperage (which=more play time) but the voltage will remain the same. you might chech on this at a local audio shop to see that its done correctly. heres a link to an audio forum, its about the biggest and best around IMO