The most common fix that people seem to do is to permanently seal the third brake light assembly to the back of the cab with an adhesive such as glue or urethane.
F150 third brake light leak fix.
Removing the brake light.
Fixing an f 150 third brake light leak replacement gasket.
Do not do this.
I bought this peel and stick variety and it worked.
Does your back window leak in your f150.
Once you do this.
Start by removing the two screws holding the brake light to the cab.
Lift the housing away and twist the light sockets out of the brake light housing.
Examine the foam rubber gasket where you saw bubbles and you may see why it was leaking.
Unscrew the two mounting screws visible on the front of the old brake light.
The only part required is a replacement gasket.
The fix is simple and will only take about 20 minutes.