I think I have a good explanation for how water got into the roof, it is a big truck cab that had clearance lights, or a least what was left of them. the lenses were missing, the bases were almost totally rusted away, and the seals were badly dry rotted. I had originally thought that the most likely culprit was a mouse nest, but when a couple of you mentioned water getting into the roof I thought about the clearance lights.
The rust could probably be patched the conventional way with a few smaller pieces, but I am stiil thinking the easiest way to change it over would be to transfer the whole roof panel over from the other cab, since there are no shaping or fitment issues anyway, and It seems like it would be easier to hide the welds in the factory seams.Since I started this thread, i have done a little more research and found out that you can remove the roof panel by cutting out the spot welds in the drip rail. My thinking was that I could take this procedure one step further a remove the inner roof panel as well, and then reassemble it all with the good inner panel from the old cab. I may be dreaming, but it doesn't sound that hard to me, and to some one with the amount of welding (very little) and body work (none) experience I have it seems less daunting than welding and grinding seams in a very visible area. It also has the added advantage of allowing me to clean up any potential problems lurking in the roof. Unless I here from you all that this is an absolutely horrible idea, I am going to try to dissasemble my old cab in this way, So that I can get a better feel for how well this might work.
It may sound like i am aswering all of my own questions here, but I am just trying to organize my thoughts a little better. I was in a hurry when I wrote the first post, and it wasn't as clear as it could have been. I am still very open to advice and suggestions. I may very well end up using the conventional method, welding small patches and using a grinder and bondo to clean it up, but I want to make sure I have looked at all the options.
