When the top is closed, I get a warning that the top is not closed, yet everything seems as it should. Which switch is the likely problem.

Btw, all is fine when the top is open -- no warnings.

Thanks

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2008 S550
2010 GLK350
1997 E420 (retired)
2005 CLK500 (retired)

The most direct way to tell is to use a Mercedes scan tool and look at switch status.

If a scan tool can't be found, you need a wiring diagram and location of top control module then test for switch status at the top module with a multi meter.