I recently installed a Curt hitch on my jeep wrangler. It is perfect, but I want to mount a 7-pin wiring harness via drop-down bracket. Is it safe to drill and tap the main bar that goes across the back of the jeep? I feel like it is sort of sketchy to drill the main bar for the hitch, but if I fill the holes with hardened bolts (probably 1/4-20) would this still be safe to tow with at the maximum capacity (4000lbf)?
I don't know what the Curt hitch looks like on your Jeep, it's capacities, or anything like that.
But, if the hitch is made of 2-inch square tubing, you can safely drill a 1/4-20 hole in it.
Drill through the center of the square tubing, though - don't try to get crazy and drill long ways through a full side of the tubing. That would obviously compromise the strength of your tubing.
If you plan to keep the Jeep and hitch for a long time, hitting the exposed metal parts with a little bit of paint will prevent it from rusting faster than normal.