Well, since that didn't work, the fuse block is next. I don't know the layout of your fuseblock, but on mine the back-up lights are shared (#5) with the blinkers, fuel gauge, brake light, handbrake warning light and the speed increasing solenoid. Do you have any other electrical components that aren't working, besides the backlight for the gear selector?
Another possibility is the back-up light switch, on the transmission itself, has gone south.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.