After further investigation, I have 4 lights not illuminating although the bulbs and sockets all test ok. The bulbs are all single filament brake lights. All the dual filament bulbs and the high mount bulb are working fine. I haven't replaced any bulbs since this problem began so I now thats not it. Also, the "brake lamp" annunciator is out now. I still think its the safety sensor but haven't gotten around to replacing the yet......

 

Finally found the fix...pulled out both wire sets for left and right tail lights. Never noticed it before but there is a third little light that faces sideways and found it burned out. Also, one of the dual filaments on the L brake light was intermittent so i just replaced both left and right. Also found corrosion on the pin connectors (probably form bad tail light seals) so sprayed them down with contact cleaner and packed all connectors with dielectric grease, put back together, good as new. Hope this helps someone in the future...

 

It's nice to know the resolution of a problem.

That warning light compares the wattage from side-to-side, and corrosion in the bulb sockets can influence that measurement. Sometimes bulbs from different manufacturers are just enough different to set it off.