I personally found the rubber duck antenna to have no use on any moving object. In my 25+ years as a commercial truck driver I have seen various types of CB & AM/FM antennas. Conway Western was too cheap to order AM/FM radios in their tractors for many years, so we line drivers had to bring our own. Many drivers used Rubber Ducks, mounted on the right mirror bracket, for AM/FM radio. Their reception was poor compared to my spring - mounted steel automotive extendable antenna, with separate ground wire. My CB antenna was a single Francis mounted on the cab rear, up high and using the cab roof as a ground plane. Grounding was poor on the Ford tractors we were issued, so lots of ground wires were used.
I will echo the comments of others here about proper grounding being essential, especially in today's plastic cars & trucks. For my Uniden scanner I use a Larson magnetic mount on the center of the cab roof of my pickup. As Dick mentioned, I use a drip loop before feeding it through my sliding rear window, then feed the extra coax around the back of the cab and floor to my scanner. The roof scratches may actually help the grounding, as well I have run a separate ground to the scanner bracket.