To get the rear seats out, you'll need to remove the bottom cushion, remove the four bolts at the bottom, then it slides up and out.
As for the wiring, open it all back up and check the wiring. Either your new player shorted something or you knocked some wires loose.
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'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne