i have the same trouble, that i am trying to fix. i am using a water hose on low preasure and pointing at side markers ( birds )
tail lights, letters , bumper bolts one at a time. i have new gaskets on the tail lights , and side markers and both are leaking.

i am working with calking strip " dum-dum " trying to stop the leaks i found. i am trying to use the gaskets also with the dum dum.

getting water in tail light housing you may try the water hose
at low flow to find the entrance area.