Weak ground or weak power connection?
With the bulbs in place, measure voltage again and see if you still have 12V with a load.
Use a piece of jumper wire to ground the light at an alternate location to prove a good/bad ground.
2005 2500 Cummins/48RE/3.73, QCLB, 4wd, BigHorn, Edge Juice w/ CTS + Turbo Timer,Transgo Shift Kit ISSPro Oil and LP pressure gauges, GDP 20/2 filters, Custom Diesel Steering Box Brace
'10 Forest River Shockwave Toy Hauler 21'
Honda EU3000I Genny