I know ZIP about stereo's but is there any reason you couldn't attach the board to the back of the seat? if clearance is a problem put a hinge at the bottom so you can flip them down to get to them or angle them out toward the top, should give you an extra foot of room. Or do you have the doof doof's in front of them?

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Beaten up old 98 Gen 2 Liberty, 13.0, stock V3 STI running gear.
07 Transit motor home
69 VF & 70 VG Valiants needing full restoration
Yamaha Zooma 50cc rocket hopefully going with the Transit.