I measured across the bulb. Tried to run a find a spot on the frame for a good ground, but didn't have anything with me to get the paint/rust off. Didn't have 40 feet or so of wire either to connect to the battery.
This bolt probably won't turn off anyway. Have to cut and drill it out I imagine. Maybe before I do that I'll cut the insulation back on the wires a little and wrap a wire around it and bring it to the front for a good ground and see if it makes a difference before I start cutting bolts. Leaving town in a couple weeks and don't want to make too many fixes before I hit the road:) Thank you for your help in identifying the ground for me.