my drains were also clogged and not hooked up right ( that one is on RAM!) after fixing that i still had dampness and like you I replaced the third brake light ( once again - so original plus two replacements) but this time I sealed it as i said earlier with a flowable silicone for windshield...
where are you purchasing your oil?
Also is you headliner water problem from your third brake light?.....if so i solved that by using a windshield seal flowable silicone around the third brake light..stopped it good
this s a major problem (truck should not be driven) and its not fuel related it is mechanical and probably needs to be repaired at a dealer..since ram has an issue out on this
problem - not sure what the code number (is it P0336? if it is - it is a Crankshaft Position Sensor A Circuit...