Is the Radio mounted to the bottom of the dash were the bracket makes good contact to the metal?
Make sure the tire carrier has a good ground, check it with a ohm meter....both open and closed.
Make sure the Mount is making good ground to the carrier, the paint on it is thick.
I assume you have checked the SWR on Transmit and it is good, if not it should be checked.
If you can unscrew the antenna connecter from the back of the radio, and high noise levels are still there, it is comming in through the power cord, Magmun makes a good power cord that is filtered, or you can home brew one. If the noise is gone, and comes back with the antenna hooked up, that is a little harder, need to stop it at the source.
As Doc asked, is it with the motor on or off?
If you are getting a bunch of noise with the motor off, you might have a radio problem.
I run a President Lincoln on Ten meters in a 1985 full size Bronco, with a 10-K antenna, about 16 foot of 8M (cut to fit) antenna is mounted to the tire Carrier, and it works very well.
If your bronco is noisey (mine is, 3" dual exhaust dumping out the side, just ahead of the axle,Flowmasters) it will help to run an extension speaker If you are running the volume wide open to hear it .
Some of the smaller radios have an even smaller speaker in them, not good.
Let us know the answers to these questions.
73
Jeff