Remove the taillight
Now you can access the old gaskets
Peel the old gasket off. It’s very sticky in some areas. Be careful not to break any of the plastic rods. Put the new gasket on.
Replace the taillight and secure it. I did my other taillight also even though it didn’t leak. Secure everything back in the order you took it off. After everything is back on, run water through the taillights again to see if it continues to leak. If it does, I don’t know what to tell you :/
BUT it worked for me so it should work for you