I have been repairing rust on my rx 3door today and when I pulled the tail lights to cut out and weld in new metal I seen that reason the quarter panel was rusted out is that the tail light on the passenger side was leaking and cause the taillight housing to rust out. What type of sealer or glue do you guys recommend to seal the taillight back in?

I would recommend using 3m butyl tape as its pretty much what was there from the factory.

 

If you're planning on removing the tail lights and reinstalling easier, I typically cut the inner most hole open so I can slide it in. Most the time, the screws that hold the garnish with the license plate lights on, don't like coming out and cutting the hole open makes it a lot easier to slide the light in behind it.