I've been having this for a long time. I don't just have noise I have blindingly loud static, from the ignition system.

Why? not sure. It's all been replaced recently, but I don't really care about AM signal anyway.

When I get around to it I will install a power line conditioner - basically a serious of capacitors. It installs in-line with the power to your radio and *should* eliminate noise.

Doesn't treat the cause, but should help the symptom. I have a link around here somewhere.... the only critical bit is that you need to make sure it's rated to give out the peak power your radio requires and I don't know what that rating is. I should think that it wouldn't exceed the fuse ratings. So not more than 15A*12V... 180W peak power and ... 0.8*180 RMS = 144W. Unless you customarily run your stereo really loud, I would think that you would never need more than one that can accomodate 100W or 10Amps.
I think that you would only need to put a power conditioner on the feed for the radio, but you might need one (one closer to 140W) for the amplifier too.

AFAIK the antenna is internal and can't be cleaned. And I've had the symptoms through 3 radios (tape autoload breaks).

Good Luck,
Will

PS I found some that I was going to install for under 20$ a couple years back.
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1990 740 Turbo, on its way to stock specs, maybe beyond