Why do I get static on my AM radio and not on FM radio?

AM does not broadcast as powerful as FM, but on some vehicles there are AM and FM antennas, mostly located in the rear glass. What type of vehicle and what year?

Its a 2000 Buick Regal.

Would you say it's worst than normal for an am station?

After a short trip around the area, I find that certain power lines cause different levels of noise. I tried adj..bal, treb and base and all speakers sound alike. Th ant is located both in front and back glass with no damage or disconect.

Could there be a grounding problem? If so, where do I start? And yes, the AM is much worse in this Buick than in my other car which is older but has similar sound sys. thank you