Forget about Greenlee hole punches. Using a hole punch is just going to make life more difficult that it has to be. An NMO hole saw does a fine job in just SECONDS with no extra hastle. Honestly! I was told by one guy online to use a Greenlee hole punch as well, but I'm very glad I didn't. It would have meant totally removing the headliner in my case -- not really a viable option so far as I was concerned.
IMHO, the way to go is to use an Antenex 3/4" hole saw.
I've installed two NMO mounts on my car. I saved $240+ of labor cost (going rate for a professional installer) by doing it myself. BTW, this was all much easier than it reads!
Here is what I did:
1. I purchased the following items:
a. One Antenex 3/4" hole saw. http://www.radiooutfitter.com
b. Two Antenex 3/4" NMO mounts with 17' RG-8X coax. See above.
c. Two Antenex NMO mount rain caps (for when no antenna is attached to mount). See above.
d. Several 3/4" rubber NMO hole plugs just in case I ever want to remove the mounts. See above.
e. One Antenex 800MHZ antenna and one Radiall-Larsen GPS antenna. http://www.ameradio.com
f. Several six-piece BNC solder clamp connectors for RG-8X. About $2 each. http://www.wb0w.com
g. Coax stripper for RG-59 (same size as RG-8X). Quality one, not cheap one. Radio Shack.
h. One small pizza pie pan. Target.
i. One short metal fish (fiche?) tape. Home Depot.
j. One small stainless stell prybar (looks like a miniature crowbar). Target.
k. Masking tape. Target.
l. Black split-loom to protect the coax in certain locations, such as transitional areas and
under the dash. Home Depot.
Already had wrenches, socket wrenches, star point wrench set, plyers, drill, soldering iron, solder, household vacuum cleaner, etc.
2. Gently rapped my knuckles down a line from the middle rear of the roof to the middle front of the roof to detect where metal crossbars that I would not want to drill through were located.
3. Removed all the floor, pillar and headliner trim from the passenger side of my car. I did this carefully with the prybar so as not to break the trim fasteners. I also had to remove the front and rear handholds, B pillar seatbelt retractor mount, both interior ceiling light and the passenger side windsheild sun shade. Removing all this stuff was time consuming.
4. Gently pulled down the headliner on the passenger side just far enough so that I could look between the headliner and the roof to confirm crossbar locations and other obstructions (such as wiring).
5. Put masking tape down a line from back to front of the roof. Carefully measured back-to- front and side-to-side for roof locations where I wanted to place the mounts. I had to place one mount a few inches farther back from the ideal center location in order to prevent drilling through a metal crossbar. Placed more masking tape around the propective hole locations to protect the paint job. Measured and remeasured. I did not drill until I was happy with my propsective hole locations.
6. Drilled two holes in the pizza pan, threaded the fish tape through and tied the tip of the tape to one side with wire.
7. Gently shoved the pizza pan up above the headliner and below the roof so that it sat under the location where I was going to drill. This was to catch the hot metal plug and hot metal shavings that would fall down while drilling. I didn't want this hot stuff burning any holes through or melting my headliner, nor did I want a bunch of *^&(*&( metal filings stuck up there.
8. Drilled the holes. I used low RPMs with some pressure on the drill bit. Piece o-cake! Took maybe 10-15 seconds, if that. Contrary to Antenex recommendations, I did NOT use drilling oil. (I didn't need to extend the life of the hole saw and didn't need an oily mess either, thank you.)
9. Vacuumed up the metal filings. Gently pulled on the fish tape to remove the pizza pan.
10. Installed the NMO mounts into the holes. The mounts already had to coax attached. I put the coax through the holes and temporarilly out the passenger side of the headliner first, then dropped the mounts themselves into place once the coax had all gone through. I had to be careful to keep the mount centered while screwing it down (from the outside. I also had to be careful to keep the mount from rotating while I screwed it down. I did this by using a felt pen to place a registration mark on the mount base and the car roof (to detect rotation) and another mark all the way around the mount base (to detect mis-centering). Then I tightened it down with a wrench. Note that the NMO mount has an o-ring in the base to seal it against leaking.
11. Ran the coax above the headliner down the A and D pillers (my car is a wagon with A, B, C and D pillars). I had to be VERY CAREFULL not to disturb the passenger side airbags which are in the headliner! Had to be VERY CAREFULL not to run the coax too close to those airbags. In the case of the coax running down the D pillar, that was not a problem. In the case of the coax running down the A pillar, I followed the existing AM/FM antenna coax pathway. (My AM/FM antenna is the Euro style mounted on the rear of the roof.)
12. Ran the coax down the A pillar to the dash for one antenna and under the door trim up to the dash for the other antenna. I put split loom over the coax wherever I thought it might be exposed to heat or any possibility whatsoever of abrasion or wear.
13. Soldered on and cinched down the BNC solder clamp connectors. Unless you've invested in a high quality crimper and learned proper crimp technique, I heartilly recommend the six piece BNC solder clamp connectors over crimp connectors. They're very easy to put on, solder and then "clamp." They're also removable and re-usable. If you mess up, you can simply repeat until you get it right.
14. Connected coax to radio and tested.
15. Carefully put all the trim back into place, etc. This part took a long time.
When I sell my car I will remove the antennas and place the NMO rain caps over the mounts. I will tell the buyers (as a selling point) that the car is both GPS and Cellular antenna ready! If I find that buyers don't like the antenna mounts, then I'll remove the mounts, place the rubber 3/4" NMO hole plugs in their place, and say nothing about them.