No Signal Strength - Mobile Antenna Install

I have what I hope is a simple problem, but I can't seem to wrap my head around it. A little background;



I installed my scanner (Uniden BCD536HP) in my vehicle, using a 3/4 hole NMO antenna mount. I have a Laird antenna on it at the moment, but have also tried the Larsen tri-band.



When using the antenna that came with the unit, I get a signal and it scans fine. When connecting the antenna in the vehicle, I get no signal at all; no signal strength meter, no DATA lock, etc.



I check the antenna cable / mount - no shorts between center and ground, continuity to vehicle ground and tip to tip is good. I am clearly missing something but I'm at a loss.



My next thought is to connect the antenna to another mount to test the installation, but as I don't have shorts or opens, I doubt that will show anything.



Any ideas out there?

No Signal Strength - Mobile Antenna Install