I have installed an aftermarket satellite radio in my commuter car ('92 Accord), which plugs into my lighter. When I have the fan on for my heat/defog I get quite a bit of Radio Frequency Interference. When the fans not on there is no interference.

I have tried using a Radio Frequency Filter on the power cord from the lighter to the satellite radio, but that doesn't work. Also, I have put a filter on the cables coming from the satellite unit and that doesn't work either.

Any Advice would be appreciated,
Thank you.

Let's figure out where it's coming from.

What I'd like you to do is to basically encase as much as possible the wiring and the back of the GPS unit in aluminum foil. Bunch all the wiring together if you can. Ground the Aluminum foil if you can.

Test.

If you have a way to power the GPS from an indoor power supply, take an extension cord and power it in the driveway without the Aluminum foil.

Test.
Move the GPS wiring and unit as far from the blower as possible. See if the noise is noticeably different or less intense.

I'm thinking that it will fail all three tests.