A knockout punch will distort the sheetmetal surrounding the hole to some degree depending upon how sharp the edges are kept (some knockouts leave pretty large dimples around the hole when used on thin gauge metal) and a 3/4" hole might seem awfully large to some guys to just run lighting wires. You'll also have to disassemble & reassemble the headliner & upper moldings to gain access to the backside of the sheetmetal roof. let me perhaps suggest that a small 1/4" hole drilled anywhere else will work just fine. Holes in the sheetmetal are easily sealed with silicone and holes in the cap can be easily repaired with fiberglass resin. I've seen numerous guys who simply used the plastic-sheathed spade connectors (and even trailer harness connectors) and just unbolted the top, pulled it back a few inches to run the wires before sliding it back in place and being done with it. By using connectors you can easily replace the sections of wire that run thru the cap joint without having to splice them but even if you don't a simple wire-splice is no big deal. Plenty of guys have just run the wire from the light, thru the joint between the cab & the cap and never had a problem. IF the wires aren't thick and are coated with silicone there often isn't any leak at all. Sometimes there IS and you wind up chasing it down but that's the cost of not drilling holes in your truck. There are a number of ways to run the wiring, it really depends on what you want to do and what you're comfortable with.