I have a 2005 Dodge Neon with a stock 120 amp alternator, an external voltage regulator, the Big 3+, and 1/0 gauge wire ran from the under hood battery to the back at my distribution block. I too had an extra connection not being used so I wired up two Dekka 98 amp/hr batteries under the sub box in the spare tire well(they each have their own 120 amp fuse). There is a Hifonics amp clamped to 1300 watts running the subs and then a 4 channel amp running everything else.
With the engine at idle the voltage dropped to 13.7 volts when it registered a 137.6 dB on music.
Oh, I also have a constant duty solenoid wired up on the 1/0 gauge wire going to the trunk. This is to isolate the starting battery from the stereo batteries.