Here's what I found in my ''85 when I had the same problem

1.  Cheepo factory stereo for that year used a common ground for both
channels.  If only one is inop, then the signal from that side is bad.

2. If  you replace radio, you will need to run a separate speaker
'ground' (negative connection) to back of van

3.  Take the speaker down (I'm assuming you have a passenger van with
the hanging plastic speaker boxes).  Loosen the screw and slide off of
bracket.  Careful not to pull, or you will pull the connector off the
plastic housing (been there, done that).

Get your trust Volt Ohm Meter and some wire.  Find that channel
connection to speaker behind radio.  If you are lucky, you should be
able to remove the ashtray and access the 3 pin multipin connector from
on top without moving the radio.  Disconnect, and on the speaker side,
time to check continuity.  Note, the wires are color coded, so the color
code in the back of the van should match.

If you may want to just connect another speaker at the connector at the
back of the radio to see if you lost the channel on the stereo

Hope this helps, feel free to email with specific questions