Had a similar problem with a 94 wagon we had briefly. Turned out to be a frayed power wire for the old analog phone. It was routed under the passenger side carpeting, direct from the battery, then thru the console to the phone. It had worn from the constant movement of the carpet, wearing enough to short against the sheet metal. Even if you don't have a phone mounted in the car, an old harness may still be in there. Just a thought.

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1995 MB E320 Wagon
2001 BMW 530i
2007 MINI Cooper S (!)