I have a small problem with my 2001 f350 4x4 dually 4door,7.3L automatic. its a farm truck and I have had some problems with rain getting into the cab and into the under dash fuse box. I am working to find the leak but not sure about source anyway here what I have.
when it rains slow or with snow melt it would get wet inside the under dash fuse box. we would let it run and turn the heat up to dry it out. everything would work fine after fuse box moisture was removed. this has happened 4 times the last time everything works except no low beam on the head lights. here is what I have done.
replaced dimmer switch with new one
replaced under dash fuse box and module with new
check all fuses
under hood fuses for low beam are not getting power, I checked and all the wiring is good to the fire wall and I jump the hot wire and all is good under hood and low beam works.
the problem is under the dash between fire wall connection and dimmer switch.
the dimmer switch gives power to red/yellow stripe wire leading from switch, under hood it goes to red/blk stripe.
I was wondering if the dimmer goes through the under dash fuse box etc. or directly to the under hood box.
any Ideas/help would be greatly appreciated or website were i could download wiring diagrams etc. thanks don

 

The windshield could be the source of your water leak - pretty common location for problems. Not sure what to suggest on the wiring issues - you've done the obvious stuff.

 

X 2
I agree

 

The leak is either coming from your windshield, or your cab clearance lights. You need to fix the leak before you replace anything. Utherwise the water will just ruin the new parts you have replaced. If you replaced the fuse box, did you replace the GEM Module at the same time? Your GEM Module is probably bad also.