There is moisture getting inside the clear part of the tails (with the signal/reverse lights), but the outer red area (taillight part) looks fine. Obviously they are sectioned off, but where is the moisture coming from? I want to seal it up but I'm not sure where to look.

 

I would take the tailights off and look. You should be able to find something that way.

 

i would just let it dry out, and run a clear silicone sealant around the old seal. just to be safe.

 

check if your trunk(where the spare tire is) has water, if yes then your tailight seal is degraded. Get a new gasket and that should solve your problem, oh and u may need to use a hair dryer to get the moisture out of the tail light