Not sure I completely follow what you mean but if I am I can tell you the way I have done it in the past. Kinda ghetto, but it worked and kicked!~!
I had a pair of 15" subs and a bose amp, Im not alot into car stereos, since I cant afford that and my computers so youll have to pardon the bad terms here...
I needed a way to power the amp for the subs, so I bought a battery charger that plugs into the wall. I tried doing a search so I could find a link to show you what I got but had no luck.. hmm..
Basiclly it plugs into the wall and charges the battery for the car.. It has a wall plug in, and has 2 of those pinch connections that go onto the battery. I plugged it into the wall and attached the pinchers to a wire and wired it straight into the amp. I was able to power the amp easily and it worked great... I was able to control the amount of amps and all that that went to the amp so I wouldnt put to much into it... Like I said... its generic, but it worked great!~!
Let me know if you need any more of an explination and Ill see what I can do for yea...
Luck
IFMU