Viper 791xv, 2003 Mazda 6

Common sense physics...  Your wire harness is absorbing too much of the shock before it gets to your sensor.

Screw that thing TIGHT into the inside firewall.  I'd suggest central location, perhaps just to the right of the gas pedal.  You want to mount it to the frame or some part of the body that is FIRM. 

Think of it like this:  If you WANT vibration to travel from the point of impact to the sensor, you want a SOLID surface from the point of impact to the sensor.  You don't want to put a "shock absorbor" between the point of impact and your sensor.

If you wanted to feel every bump in the road you would remove your struts and springs on your car that absorb the bumps and shock from the road. 

In your rear end's case, you WANT shock absorbtion to keep your butt from getting numb.

In your sensor's case, you DO NOT WANT shock absorbtion - you want your sensor to get all the shock it can to increase sensitivity and than you can adjust the sensitivity DOWN from there if need be with the controls on the sensor.