I have a very low mileage 2004 F150 and live in the Seattle area. I have noticed water accumulating on the left rear mat and seat support area. It does have a sliding rear window (and yes, it's been closed...). It is extended cab with the rear half doors. There is water in the channel where the wiring runs from front to rear. Any help on where to start looking is appreciated.

 

Check the third brake light, 2 screws on the outside. The seal is known to leak. Some rubber silicone works, or go buy a new light. Pop a grommet in the wire channel to let the water drain out the bottom.
Welcome and good luck.

 

Thanks for the assistance. The gasket for the third brake light was not applied correctly from the factory. I called the dealership in Everett, the parts department was of very little help. They wanted to sell another light at $75 when all it required was a gasket. I talked to the service manager and he said that this is a huge problem on these models. I went to home depot, bought some automotive style universal weatherstrip material, cleaned up the light, applied the material and installed it. For extra measure I put some silicone in the clean gap between the lamp and body (I made sure there were new light bulbs in the assembly first.

Thanks for the help.