Well, I never post on here, but I figured I'd chime in with the system I dropped in the Caddy.

2 4" Diamond Audio M3's up front
2 6x9" Diamond D3's in the back
2 12" Cerwin Vega IT subs in the trunk, sealed box
Some 7 year old ass Alpine CD player that I've had forever
MTX Thunder 6500D amp for the subs
MTX Thunder 6304 amp for the mids and highs
Optima yellow top
All stinger wiring, 150 amp fuse
12ga to the subs, 16ga to the cab speakers
4ga power and ground from battery to battery
8ga power and ground from rear battery to amps
18ga remote and common ground

It sounds good and it's pretty fuckin' loud.