* '92 124 Wagen. You're saying that with the doors closed, including the hatch, the four courtesy lights, one in each door, all stay on at the same time? I assume that the overhead light for the hatch doesn't also stay on.
* The only way that this sounds possible is in the unlikely event that all four door switches broke the same way, namely never allowing a disconnect in the respective circuit. It's even not that easy for all four lights to NOT work at the same time (assuming that all four bulbs and the power supply fuse are good).
* What do you find when you check power and ground at each door switch? Disconnect/reconnect power at each switch (the switches come out easily), one switch at a time. Does that always turn off just one light or all four?

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ASE Master Tech
Quality Enterprises
Mercedes Svc. & Restoration
Sheridan, IN
317.769.3536