How do you fix the tail light on a 1972 volkswagen beetle after the correct bulb and fuse have been replaced?

Did you check to see if you actually had power to the light socket? Old cars are bad for having bad grounds.

Check and clean the light "Housing" connections first if you have power to the socket. The housing and body will sometimes corrode just enough to lose the ground. Just loosening and re-tightening the bolts will some times fix it. When you check for power make sure you use more then one place for a ground. Make sure the fuse connections are nice and clean as they can get a real fine layer of corrosion and then you won't get a connection there either.