Yes, the rubber gasket is available separately from your local dealer. Should run about $4 - $5. To replace it, you need to first remove the outer cover from inside the car. Use something broad, flat and relatively short to carefully lift the tabs on either side and pull it away toward the front of the car. To remove the light unit itself, locate the wide, flat plastic tabs (two of them) and flex them away from the light housing and then pull the housing away. The tabs have a ratchet-type engagement, sort of like an electrical tie-wrap, just bigger. It should be pretty self-evident once you're in there. Clean the glass, install the new gasket and reinstall in reverse order. If you break any of the pieces (and they can become brittle with age, so be careful), you'll have to buy the whole assembly for about $50.

Hope this helps,

Gary