Wanted to post my journey to repair my intermittent backup lights. Doing my part to pay folks back for their help on this forum.

Symptoms:
- Sometimes worked okay, other times nothing
- Sometimes moving the shifter handle forward a bit worked

General info
- Two years ago I had replaced the start inhibitor/back-up light switch (the wedge to the right of the shifter in its housing). Aftermarket. Not Volvo
- Performed the standard inspection of fuses and bulbs

Fixes (yep as in more than two in my case):
- Figured the switch was out of adjustment so I tried that. No lights whatsoever
- Pulled the switch out. Not good. Two wire's soldered joint's had failed and another was on its way. Re-soldered all the connections
- Retested. Lights are now intermittent
- Opened the switch and cleaned the contacts. It did not look good in there for a 2 year old switch.
Retested. Better. Lights are still intermittent then won't work at all
- Insert your favorite expression. Charlie Brown's was "RATS!"
- Went to the light assemblies, took hold of the connector and "Let there be light" without removing. Wiggled them some more and found a third issue
- Pulled the connectors and found some corrosion. Corrosion-X to the rescue (love this stuff).
- lights are now fully operational

All the best.