...damage to either unit? I have a 2003 Sierra with a stock am/fm radio with RDS that uses the speaker system to transmit information signals, like a chime tone when the fuel is low. Aftermarket head units would require an expensive special adapter to connect if I want to retain that function. Couldn’t I just install another head unit with a CD player in the truck, and share the stock speaker system? I am concerned if feedback from one unit to the other could cause damage or other problems.

In theory, you could connect them to the same speaker system, but you would want to put some sort of switching system to prevent any sort of grounding or noise problem. By the time you install the 2nd head unit and wire everything correctly, you'll have thought twice about just buying the simple expensive connector.