License Plate Lights on 1998 Ford Expedition

Have noticed license plate lights were out. Replaced bulbs, still no lights. Checked fuses, all ok. Tailights are operational. A set of rear back up lights (installed aftermarket) are also not working. Replaced these due to corroded state and still both not working. Replaced relay that comes with fofg light set, still not working. Looking for next item to check.

Asked by for the 1998 Ford Expedition
I have had the same issue and solved it by cleaning the ground wire.