Lighting issue
 

So I tried to search but seems most issues aren't the same as my scenario.

2006 Ram 2500 5.9

had aftermarket tails when I got it, terrible fit. I got new aftermarket tails and installed.
All seemed well, brake lights work, turn signals work, running lights work.

Now the issue, when out into reverse, my tailgate light bar works in reverse as does my passenger tail, drivers side shuts out completely, no brake light no reverse nothing. Until taken out of reverse.

Question is, where should I start to diagnose this?

Your info is a bit sketchy, but I would start by checking the wiring and from that make your own schematic. From that you should be easily able to see if something is wired wrong.

it ended up being the wiring harness, weak connection.

Thanks for coming back and letting us know what you did to fix the problem.