Might also check with the car manufacturer as often this type of problem is addressed in a technical bulletin. While I have a couple extra birdies due to the car (Believe it is the fuel pump) the rest of the spectrum is ok. Makes me wonder if you have a loose ground somewhere. What make/model might help too.
The external antenna is a good suggestion as it gets the rig away from the noise which is usually low power. If it helps, you might move it around inside the car and see if you can isolate a source for the noise. Then again you may just want to listen if t really helps.