Very little to no sound coming from the speakers in my 1989 Cadillac Eldorado, but the radio has been sent in and verified good. Is there a relay for the speakers that could be causing this?

 

Most likely the amps which are part of speakers. There are two fuses and relays that power amps.

 

I would check the amps next more specifically, the remote on wire, ground, power and fuses. use a multi meter or test light. if those all have current you need to check the feeds (the left right front back and center channel supply wires) -- i am assuming you know for sure the speakers are not blown? just one could foul out the entire system causing a short.