Ghosts in the machine?

[71tahoe]

I have a strange problem when it rains really hard, the parking lights come on. It would be a great safety feature going down the road but it tends to run the battery down sitting in the drive. There are some leaks in the cab but I checked today and didn't find any water near the switch and the fuseblock was dry also. The strangest thing is that with the switch in the off position (pushed in) I could dim the dash lights. I suspected water may have gotten in the switch but finding it dry has me puzzled.

Any ideas?