Same here - AM works fine with Accessory only, but when the engine/IMA is running it is full of static.
Best that I can tell, RFI could come in from the front end (antenna) or the back end (power supply & ground). Radio is an Alpine, maybe 5 years old.
There is not much on line about AM antennas that perform super well...... on some Ham sites there are posts about wavelengths and antennas, and it would seem that at 1.0 MHz, the antenna would be about 300 meters. You could base load this, but it would hang out in the air and be a real drag.
There are filters available for conditioning the power line in, but I haven't looked to check connections, grounds and all that yet. Some day.....
Thor