A modern car has a lot of electronic "smog" and my experiance are that it mostly appears in VHF and LO-VHF.
Yep - in some of my mobile installs, both the Unidens and Whistlers get hit hard by the junk emanating from the dash.
One solution, with antennas in the rear of the vehicle, is to use #43 type (30-300mhz) ferrite chokes near the input of the scanner, AND near the feedpoint of the antenna itself.
That got rid of much of the common-mode interference hitting the coax braid, and eventually traveling back down inside the coax to the scanner.
Mag-mounts are particularly prone to common-mode interference, so we drilled holes (gulp!) for a good quality nmo-type mount.
Still, in stubborn cases direct reception by the antenna itself, and not the common-mode problem of the braid, can lead to head-aches no matter what.