1964 rear lights
 

Had a cracked drivers side lens. To change it the whole tail light assembly has to come out from the car. When they put my car together 20 years ago, they never used a gasket between the body and the tail light extension. while apart I regalvinized the light housings, when together and the lights on , the running and brake lights were noticeably brighter. Put new gasket on and fresh donuts on the studs also .

That could be the reason why your lens cracked.

Not the cause. It has a lens gasket and a gasket around the bulb housing . the gasket that was missing was the one between the rear body and the tail light housing. the hair line crack was there for a while, just lazy cause from start to finish it took 15 hrs between both lights