I realize that my background is in 2.4GHz, but even in the 100MHz range I would not expect it to be acceptable behavior to use 2.5mm jacks and standard wire to carry signals, or to attach anything to the end of a preexisting antenna. I would try to find some sort of cheap adapter or something with a socket you can cut off that will accept your car's antenna connector. Preferably something with a few inches of cable still attached since I'm not sure what impedence coax cable to recommend you use. Remove the stock antenna and attach the center lead of the new socket in it's place. Run a wire from the nearest ground location to the shielding of the cable.