Why are my back speakers not as loud as the front

The relay is only used with the Infinity system, and it affects all of the speakers. There are little amplifiers bolted to the door and lift gate speakers that get their power from the relay. Those amps do not increase volume or power. They only change the tone response for the specific vehicle.

If you have the basic cassette radio, the amplifiers are not used. Chrysler radio output ICs give such little trouble, it's more likely you do have the Infinity system and the rear speaker amps are defective. I used to repair them for new and used car dealers because replacement speakers cost as much as $145.00.

I'd start by popping off one rear speaker grille and removing the speaker to see if the connector has two or four wires. (Pry the bottom of the grille out. They have fingers on top that you don't want to break off). You CAN use regular speakers in place of the four-wire speakers but finding the correct shape and size will be a problem. You'll also be able to find the correct two-wire speakers in most salvage yards. Since two radios gave the same symptoms, it suggests the speakers are the problem.