I had to replace my CB module - although it's got a little rubber sealing strip between the lid and the case, mine had not been properly fitted in the groove for it from new, and it let in water. The position the module sits in is ideal for water to eventually swamp a bit of the board and blow it. Initially I dried and cleaned the board, then closed it up with the seal fitted properly, and that lasted a while, but eventually, although no more water had got in, it failed.
Although mine is a Rushmore, and the [different] module is located in a different position to pre-Rushmore models, that could still be your problem I guess. The good news is that pre-Rushmore CB Modules are plentiful and cheap on eBay. (Rushmount-era ones are decidedly NOT cheap!)