[Barnsnake]

Hey Glyn. I'm just fine. (My car is all dry and clean again.)

How close is too close? I'm not sure. I've never seen a problem quite like yours.
Are you using a separate amp for the speakers, or is all of the power coming from the radio itself? Is the speaker wire bundled with the sender wire as it goes to the rear of the cockpit? (I'm assuming the speakers are in the rear bulkhead.) You could try running a temporary set of speaker wires "cross country" over the cockpit to the trunk to eliminate the harness induction possibility.

You might try putting a piece of sheet metal between the speaker and the wire as a temporary test. If it eliminates or reduces the needle wag it would confirm induction from the magnet as the culprit.

Keep us posted.