Jeff,
I had a similar problem where my external amplified speaker would squeal when I transmitted. Some of the previous responses suggest that the problem is on the 12v DC side of the system (between the battery and the transceiver) and others have suggested that it is on the RF side of the system (between the transceiver and the antenna). In my case, it was a combination of things.
The first thing we need to do is to determine whether the interference is originating from the 12v DC or the RF side of your transceiver.
1. Does the problem occur when you transmit on low power? Does it occur on both 2m and 440? Do you hear any noise on your stereo before the protection trips?
2. Temporarily run a pair of wires directly from your battery to your transceiver (bypass the distribution block in your console and put a fuse on the power lead). I have a hunch that the problem is not on the 12v side, and you will still trip the protection on your stereo
3. Disconnect your antenna cable at both ends and buy or borrow another cable and run it directly between your transceiver and your antenna.
4. If your stereo still trips, then make sure that your antenna mount has a good ground and make sure that your stereo has a good ground.
BTW, my problem was that RF energy from my antenna cable was "leaking" into the wire that feeding power to my external amplifier. I re-routed the power cable and the antenna cable so that they ran on opposite sides of my Jeep.
Russ
KI6MLU