THIS REALLY WORKS!
I have a 2006 Dodge Ram SLT Quad Cab.
I did the same thing snrusnak did, per his instructions in this thread.
Home Depot sells a Frost King door seal that is 5/16"x9/16" and is made of rubber, not the spongey material that is supplied by Dodge.
My factory gasket was in fairly good shape by appearance, but was failing due to the material being porous.
My 3rd light, now fits snugger and won't leak after 5 years from the assembly line.
I highly recommend repairing your 3rd light this way if you're having the same problem....
Glad it helped you
It's been a while now and still not a drop in my cab
Great post, gonna get some for my fix ASAP! Wife complains of the musty smell after the rain pours in!
Good luck (this is probably the best mod I've done lol as it keeps my truck from stinking!)