Antenna mounting issues.
For the most part, any resonant antenna "works", it's an issue of effiicency. If only 50% of the transmit power is being radiated from the antenna and the other 50% is going back into the coax you only are getting 2 watts effective radiated power - and of course people don't "hear" you as well. Proper placement of the antenna and grounding will give better performance, allowing the same 5/8 wave antenna to have an ERP of 10 watts, assuming it's a 5 dB gain antenna. Lets see, 2 watts or 10 watts, hmm...
Magnet mount antennas use induction to provide their counterpoise (or ground if you will). They are not quite as good as the NMO mount in the middle of the roof someone mentioned but are still very usefull with the right placement.
First generation folks with the fiberglass roof have 2 choices for a truly effective antenna. Either use a fender mount or body mount ala ball mount on the side of the rig, or buy one of the marine "No ground needed" antennas and mount it on the top.
I know this is kinda long haired stuff, but the antenna is really the most important part of the system. Even the best radio sucks with a poor antenna. Remember, and open differential "works" too, but how much better is a locker?