On an 84 westy, the front door speakers use a 2 wire "zip cord".  One
wire is brown, the other is brown with red stripe.  Just follow them
from the doorjam under the dash to the existing radio to determine
left and right.  You may even figure it out by removing the ashtray
and looking down from above.

The rear speakers share a common ground.  Look for a 3 wire set
entering your radio.  The ground is grey, the passenger side is blue
and grey (grey being ground "-") and the drivers side is red and grey
(grey being ground "-").

I'm not 100% sure on the red vs. blue being left or right, but you
can easily poke your head in the rear cupboard to confirm.  The grey
wire is definitely common ground.

Just make sure your new radio can share a common ground for the rear
speakers, otherwise you'll have to run new speakers wires all the way
to the back of the car.  Not fun to do, but at least you could run
higher quality speaker wire than they used in 1984.

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A fellow vanagon enthusiast showed me a neat trick, tap into the grey
with black stripe wire coming from the ignition lock for the
"switched power".  This wire provides power whenever a key is just
inserted into the lock, this way you don't have to TURN the key to
run the radio....and when you take your keys out of the lock, the
radio automatically shuts off.

>
>
>Anyway, I'm contemplating popping in a basic CD player/AM/FM radio
>in place of this fine example of late 1970's designing. Does anyone
>have a wiring diagram for the speakers?