I put a new PL-259 on, and made sure there were no opens or shorts in the cable out to the trunk lip mount. I screwed the antenna on just enough to start the threads and and checked for continuity at the PL-259. I kept checking as I screwed the antenna down. No continuity until the very last 1/2 turn, then Wham! continuity!! From what I can tell, the base of the antenna is shallower then the base of the NMO mount. so as you screw down the antenna the pin on the antenna is pushed all the way in and the brass center contact of the NMO mount makes contact with the pin and the bottom side of the antenna base causing a short. I grabbed the gasket from a NMO weather cap and put that between the mount and the antenna. That 1/16"-3/32" made all the difference. The radio is no longer deaf, and even the NOAA channels come in clear. I thought it should have had a gasket anyway.
As far as the thread difference?? On the trunk lip mount I didn't notice, but on Larson through hole, I did notice some roughness, but still went on. What are the Diamond antenna supposed to mount with?
Well I just installed a Kenwood 281 in my truck and experienced the same problem with the radio being deaf with the power on though I could hear WX channel. Took my new radio back to the store (AES) where I purchased it from, and they verified that the radio was working. Therefore this left only the mount (Larsen NMO) or antenna (Diamond NR73BNMO) being bad. I Ohm's check the cable it seem Ok but was shorted with the antenna installed, I was not sure if this correct or not.
I was searching for threads on troubleshooting my antenna when I came across this thread went out to my truck and turned the antenna a little bit and turned on my radio and it was functioning like it should. Thanks to John for finding the problem and posting his results otherwise I am not sure I would have ever figured out the problem. I will be sure to let the store know what the issue was so they can let future customer know beforehand.