If I read the wiring diagram correctly it appears that the power feed for the fuse is -
Black from light switch to the dimmer panel switch.
Tan from the panel dimmer switch to one side of the fuse.
Orange from the fuse to the dash connector pin #1.
Are you saying the tail lights don't work either?
The same terminal on the light switch feeds other lights so it's probably OK.
Dimmer switches are always suspect.
So are bulkhead terminals.
Do the turn signals indicators in the dash work? If so that should mean the dash cluster is grounded.
http://www.mymopar.com/downloads/1969/69BelvedereGTXSatelliteRoadRunnerFull.pdf
http://www.mymopar.com/downloads/1969/69BelvedereGTXSatelliteRoadRunnerA.JPG
http://www.mymopar.com/downloads/1969/69BelvedereGTXSatelliteRoadRunnerB.JPG