AM is horrendous for picking up interferance, in a car or not.
My car does the same thing, I can hear the RPM'* thru the radio on weak AM.
I even have an aftermarket stereo- Pioneer.
Of course I do not listen to AM
I will tell you just how bad AM is - They say when trying to detect electrostatic interferance, use a cheap AM radio, put it on a non-station low frequency, and hold it close to items you suspect of putting off a lot of ESI.
This is the procedure written in a computer repair book I own about when one gets weird computer problems.
Except for MAYBE using a filter in the power line to the radio, there is not much you can do.
FUN QUESTION - With XM, FM, MP3, CD,... etc, how is it that AM is still even alive?