Trouble shooting proceedures to check out Pass lock system in 1999 che

my 1999 1500 chev. express van is creating a problem with the Passlock system. ONLY when it rains. my mechanic want to buy a new module. I don't think it is that since it only occurs when it rains. I feel the problem is in the connectors that might have corrosion from the moisture. what I am looking for is a systematic trouble shooting procedure and schematic showing each connector location and pin in this system.