If someone installed an aftermarket radio or radar detector or mobile phone or etc, it would have had a lead the would tell the new unit when the lights were on. That way, it could either track the dash light dimmer just dim its own lights.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120