Remember that you could have a problem anywhere from the antenna itself all of the way to the radio. You can use a multimeter to check resistance across different parts of the system. I just put a new antenna in a couple of weeks ago, and in the process discovered that parts on the antenna were loose. Reception is much better now.
Sometimes aftermarket accessories can cause problems too. Also a bad ground at the radio can cause problems.
You may want to check each of the connecting points. There may be one under the dash towards the right side. I've never tracked all of them down.
1999 540iA Sport
Former 93 SSEi Owner
Former 89 SE Owner