How can I fix poor AM reception on my 52 plate mondeo?

I have a 52 plate Ford Mondeo. The AM reception suffers from interference from the engine, the wipers or in a built-up area. At times (eg open road, fine day etc) the reception is good, at other times (raining, built up area) it's so bad I turn off.
Is it some sort of suppressor issue?

Phil Hughes
January 2010