Was driving today in heavy rain and noticed that water was leaking between plastic panels behind the seat on both sides below the rear window of the cab. Anyone have ideas of the cause? Thanks

 

More that likely it is the rear window seal, very common issue. Probably on the side since it is not dripping from the top.
If you have a sliding rear window, that also may be the issue (and a bigger problem, ie window replacement).

You will need to remove the rear window and replace the seal (use 3M Window Weld butyl tape from any parts store, around $20 and about 1-2 hours time).

 

Check all the posts in this thread. Should find some useful info.

https://www.f150forum.com/f6/leaking...-window-45399/

 

If the rear seal is good. There is probably a Crack on the 3rd Brake Lights, Small body Crack on the Water Passage way.

 

thanks for the responses. I took off panels on one side. used my hose to try and replicate the leak. sprayed all over the back glass and the third brake light, nothing. guess will have to wait until it rains again

 

Here's a picture of the seal Ford put in our trucks' back windows.

https://www.f150forum.com/attachment...w-dsc06520.jpg

Mine was dried out and cracked more than this.