Do you mean where the body of the lens contacts the car or where the lens contacts the light assembly? Depending on which, there are gaskets to use, or clear RTV depending on which joint you mean. Also, it makes a difference where you get the replacements.
for the tail lights, theres stuff called butyl tape or something like that. Its like a putty. Its what comes with the aftermarket tail lights. But i dont see why you couldnt get away with the silicon liek said above.
i like Bapford's idea, and he obviously knows more than i. BTW, i assume you want to seal the plastic lens to the body of the light housing.
if all else fail, 3M makes a product for that and i used it once and it worked well (different application- sealed the 3rd brake light on the vert with it. old gasket was shot-was not going to buy one). just an idea.
for the tail lights, theres stuff called butyl tape or something like that. Its like a putty. Its what comes with the aftermarket tail lights. But i dont see why you couldnt get away with the silicon liek said above.