Same here. If it is a wireless FM Modulator you will get interference. They suck.
If you want things done right, you should do it yourself.
Here is a post that I have pasted to 587,986 threads just like this one:
Delphi SA10003 SkyFi XM Satellite Radio FM Modulator
Do an eBay search. I got three of these for around 10.00 each.
It is for XM radio, but the accessory/Aux plug will fit most MP3 players.
This is also small enough to hide in the left side of the stingray with the wire harness.
The positive is that it will also power your 6V device without running an additional power wire to plug the XM into.
Just a follow up to my last post:
This connects to the radio via the antenna wire.
There is a power cord for XM.
There is a cord for AUX.
Find a 12v connection that works off of the switch to plug into this way your device will auto power on.