First of all (and I know you've probably already checked this, but just in
case), see if the stereo has a fader control. If it does, maker sure the
fader isn't set all the way to the front.

If that doesn't solve the problem, pull a front speaker and plug it into the
rear connection. If it works you have dead rear speakers, If it doesn't work,
it's the wiring or the radio. Find out what color the wires are at the rear
speakers, then pull the radio out and find those color wires at the radio.
Connect the wires going to a front speaker to the rear wires. If you get
noise then, the rear channels of the radio are dead. If you still don't get
noise from the rear, then the wires have been cut somewhere. You can either
find the break in the wires or run new speaker wires to the back.

I hope this info helps.

Sean B.
'90 V'gon (as yet unnamed)