[BNaylor]

Since the replacement lamps did not correct the issue then about all you can do is confirm by checking for the proper voltage at the lamp socket and for a good ground or low. If missing which it probably is due to the lamps not working then there is an internal issue such as a lamp driver module which means radio replacement or getting the unit repaired by shop that can repair Delco radios. Other possibility could be cold solder joints at the lamp socket or on the PCB so it may not hurt to re-solder those points.