After rain there's condensation within the lenses, after heavy rain I have to drain it out

I have a DIY sealant gun, propose to run that along the joint, but not sure which compound to use.
Would domestic clear bathroom sealant do the job, or not advisable eg as it cracks in frost, or is there something better/automotive?
Also, would it be best to remove or loosen the lighting strip, or to wedge open the joint?
Advice appreciated.
