What vehicle ( SL XXX ) is this? On my 1991 SL300, the speed signal used for the auto volume control is part of the harness which is dedicated to plug into the radio head unit. Anyhow I hope it works out for you. BTW, that auto / speed volume control drove me crazy. I disconnected it. Maybe it would have functioned better if they also included in the logic whether the top was up or down.

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