First thing to check, is the coax, and it is REAL simple.... Disconnect the radio, and take the antenna off. place the leads of a V.O.M. across the PL-259 connector that attaches to the radio. Have the V.O.M. in the Ohms range. If it shows 0 (Zero) Ohms, the coax is shorted, if it has an "infinite reading ( the same as not having the leads connected to anything) it might be the antenna or it's mounting. The next test is to short the antenna end of the coax with a jumper lead, and check it again with the V.O.M. this time, you should get a 0 (Zero) reading. If not, the coax is "open", meaning it has a break in it. This sounds difficult, but it's really not. If the coax does check out OK, then again, it may be the antenna, orthe location, and mounting. You can loosen the mount, and check for rust or dirt. If it's there, then clean it, and try again. The other alternative, is as mentioned before, use a roof mount. You can use the NMO mounts which are considered a permanent mount. I have a mag mount for 10m, and a mag mount for 2m. Many will tell you this is not a good system, but has worked good for me. I have made several 10m contacts going to and from work, in the Central U.S. area from Central Texas. My 2m contacts are simplex over about 20-40 miles, and to a wide area repeater about 70 miles away. My roof is still intact, because the antennas will fall over when stuck by a tree. Permanent mounts don't allow that... Guess you can put a spring in there someplace, but then the antenna likes to lay back a LOT when driving at highway speeds. Good luck, and try to find a Ham, CB'er, or someone who understands radio and electronics to help you trouble shoot, and get it working.
Knightrider...