[ImperialCrown]

Welcome to Allpar The lens can't be completely sealed off. There has to be a vent, probably on the bottom/inner side of the lens assembly that allows it to breathe. Some have an 'L'-shaped rubber vent hose pointing down.
If that doesn't help, replace the lens assembly.

 

[L.Turner]

I had this problem on just one of the tailights. I simply drilled a 1/8" hole in the bottom of the lens and haven't had any repeats. (I realize this is a very old thread, but thought I'd add this in case someone else enounters the same problem.)

 

[ImperialCrown]

L.Turner, good to hear from you. Are you still motoring about in the PT?
This 'venting' also helps headlamps that collect moisture. You never want a lens airtight, it needs to breathe. When the bulbs get hot and warm the inside of the housing, the moisture should dissipate.