If I understand your question, you have installed the lights and then grounded them to the dash while its on the bench? If that's the case, your dash is not grounded at that point so you'll need another temp bench ground for your dash to test the lights- maybe run a wire with alligator clips from the dash to your battery temporarily. Once the dash is back in, you'll have your ground back. Additionally, you can also run a dedicated ground from the battery to the dash for accessories if you like.
CJ7, 258, Dana 20 T-case, Dana 30 & AMC 20, TFI Ignition upgrade, MC 2150 carb, YJ tub mod