There is no way for the rear seat to fold down.......the bottom is easily removed........then the bolts for the bottom retainers of the upper seat can be removed, and then the seat can be lifted and then out.....
As for your radio, we are completely in the dark on the brand, type, model you bought, or even if it NEEDS AN AMPLIFIER TO RUN.......
You can easily run wires to the trunk by removing the door sills on the passenger or driver's side, and then up through the rear seat.....
I would NEVER spend 100 bucks on an adapter......I would have just hard wired it.....if you spent 100 on an adapter, what did you spend on the radio? And how much for the amplifier? And yet you are using the original speakers? Doesn't make sense to spend all that money without upgrading the speakers.....