If you mean there was clear sealant/caulk around the tail light red lens I would
suggest clear silicone RTV. Get the small "roll up" tube rather than a big
caulking gun size as it's nearly impossible to store an opened tube more
than a very few weeks B4 it get's hard in the tube.
You don't need much for a tail light lens.
My tail lights get dirty inside from road grime. I've cleaned them twice!
I need to try and seal mine. They sure as heck aren't any where close
to being sealed the way they are made-- very cheap stuff!
Clear RTV silicone in small roll up tubes can be hard to find. You can get
the caulking gun sizes at almost any home center or hardware store.....
In fact, if you don't use a lot you could use white and you can get that