Tail Lights, Sidemarker Lights and Liscense Plate Lights do not work

I found that my tail lights and license plate lights were not working, so I first checked the bulbs, all good...then I checked the fuses, also good...on to the relays...also good (the ones that I know of).
I read that problems arise when a aftermarket trailer adapter is installed, so I replaced the one on the truck with a "plug and play" adapter...still no change.
I can't figure this one out... I even placed a second ground to the system... thinking it might be a grounding problem...no change.
I am at wits end.... do you have any suggestions?

Thanks!
Joe McGee