Tail light lenses coming off - Fix?

Black RTV adhesive silicone has always worked for me. It's not impossible to get the lenses apart if you need to, later on, it seals out the moisture just as well as (or better than) the OEM butile ribbon, and it's easy to obtain.

I suppose, if all else fails, you could either try using some of that butile ribbon sold at parts houses for windshields (very sticky, nasty, messy crap) or just buy a roll of the genuine stuff from Ford or 50resto.com.

Also, try using a pair of needle-nose pliers to bend those little metal clips that help hold the lens on so that they get more "bite" into the plastic. THOSE are what are really supposed to hold the lenses on there - the butile ribbon/silicone is just a moisture sealant, really.