I did that once to see if i could run my roof lights off it and i noticed there was a solid piece of metal there not hole to get up into roof. There was no area there that i could see water could enter. I'll try it anyway, but not today snow is covering the truck.

 

There is a small triangle shaped hole up in the upper right hand corner of the windshield. Also, try removing your visor and sticking your finger in to check for water. You may have to have someone dump water on the cab while you do this. Too bad its winter time.

 

I owned an 87 W150. I had the same exact problem. Water would be on the carpet under the parking brake pedal. Couldn't figure out where in the hec it was coming from. The firewall, cowl, gaskets on the firewall, fresh air vent all checked out good. Noticed after a hard rain that there was a slow drip coming from one of the interior drivers side far left trim screws in the upper plastic trim that covers up where the windshield meets the cab. Investigating further, I inspected the cab top front drivers side. The filler the factory used to fill the area from the cab to the lip which the chrome trim attached to was cracked and letting the water seep through into the cab. Water and melting snow tends to accumulate in there.

I took a fine wire brush and cleaned out the affected area the best I could, careful not to remove what was left of the good filler. Took some clear heavy duty silicone sealer and smoothed it in. Worked like a charm. Dry as a bone after that. The leak dumbfounded me for quite awhile.
While I was at it, I inspected the whole area of filler up there and found some additional cracks in the filler and sealed up the potential weak spots.

Hope this helps

Tim

 

What chrome trim are you talking about? I have no chrome on the outside or inside of my truck in this area. I have no headliner, and most of the trim inside is cracked and missing so ia m not sure what your talking about. Could somone take a pic of what he is talking about to give me an idea.

 

Your truck may not have chrome trim up there. Mine doesn't. What we are talking about is the rain gutter that runs above the windshield and doors on the outside of the truck. The area above this gutter is very bad for rusting out. Your cab lights could be the problem, I would seal them with clear silicone to be sure. Happy hunting.

 

See someone has unhooked and cut the wires to the lights right up underneath the lights. But i still haven't found a hole into the roof to runnew wires for them. I check the area behind the a pillar trim and the hole where you said to look is alredy filled, would there be one on the other side too? The rail is rusted but someone has already filled this up with some kind of sealant. So sealing the lights will be good then.

 

There is another hole above the passenger side of the windshiled. You should check under the carpet on that side for water too, but its less likely. It seems like you can never seal too much stuff on these trucks. I haven't solved my leaks either, and I think that the only way I will be able to is too gut the interior and reseal and paint the cab. I am planning a restoration, so its all in the future.