Troy, the reason you're having the problems is simply this.
It is very common for mag mounted antennas to have common mode current flowing on the coax cable. If you go to my web site and read this article, you'll get a better understanding : http://www.k0bg.com/gpnotes.com
The long and short of it is, there is RF inside the vehicle because the coax cable is acting a a ground plane for the antenna. You could also have a bad coax connection either at the antenna or the PL259 connected to the coax.
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