U can use this connector to feed your amp. This cable is contact switched so your battery won't drain.
You just need a bridge in your fuse box. Available from barchettaparts.
Wow this looks really intersting. Would try to make something myself too. But... The room behind the plate covered with dynomat isn't really sealed, so i'm afraid it would sound crap.
I'm thinking in placing a small sub just onder the dash where the airco is placed.
(I don't have airco in the car) This way you wouldn't even see the sub and there is room enough. But will it sound ok?