[OldBlevins]

Do you happen to have a voltometer? If you do, a good first bet would be to make sure you have a decent ground connection to the socket. Even a little corrosion in the socket can block the connection.

I haven't fiddled yet with my boxster's rear light bulbs, so I don't know, but if it's a multi-filament bulb and the running light works, it's probably not the ground. If it's not, my next move would be to use the tester with a proven ground and scrape the probe hard against the hot feed while someone presses on the brake and see if I get current.

Most of the time I have found the problem lies in the socket connections and not in the vehicle wiring.

If you don't have access to a voltometer, let me know and I'll tell you an inexpensive workaround to test your connections.