Here is the setup in my car. I don’t know which parts are germane, but I will give all the facts I think are relevant.
I drive a Prius.
I have an FM Radio adapter for an iPOD. (Griffin RoadTrip)
It was raining today and on the way in while stopped in traffic I noticed that there was a buzzing noise that got higher in pitch as the wipers got to the top of their cycle.
I briefly turned them off and the rhythmic buzzing stopped.
So, were the wipers themselves causing interefence?
Either the wiper motor, or the electronics which control it. With luck, the interference is coming in through the power line. I say “luck” because if that’s the case, it’s easy to filter it out. There are various filters for automotive power cables to use with audio equipment just for filtering out this sort of interference. Something as simple as a snap=on ferrite choke could do the trick. Stop by an auto sound shop and ask about options.
If the wipers are electromechanical (most are), then it’s also good practice to suppress the noise at source by fitting interference suppression capacitors (condensors) between each motor supply (there may be more than one) and ground. Usually these are fitted as standard on wiper motors, just as with the unswitched side of the ignition coil, but they can fail or develop a bad connection.
Well just some thoughts -
Since your radio adapter is putting out a signal, and your wipers are metal, they are causing interference as they pass through that signal.
the electrical circuit that turns on your wiper motor is sending interference to the 12v plug your putting that adapter in.
get a cassette adapter, they work much better.