Shock Sensor Fading

I added a shock sensor to my factory alarm.  I piggy backed off the hood pin sensor.  The sensor it self is attached to the steering column under the dash.  After I installed it, it worked great, I adjusted the sensitivity and tested it multiple times.  Then, after I went for a drive, I tested it again.  It was much less sensitive.  I took the sensor off, and check everything and reinstalled it.  Again, It worked great.

So what I'm thinking is that the vibrations from driving are loosening the sensor.  I attached the sensor with two nylon zip ties. 

Any suggestions on how else to install?  Thanks!