Is it possible that the third position on your dash switch is not functioning so that you really have on-off-on? My manual makes no indication that the floor lights are in any way connected to the dash switch -- it seems that they are only operated by the door switches. You might want to take the dash switch out and clean the contacts -- do a search on this forum with respect to taking rocker switches apart and cleaning them (their interiors) -- I believe I have seen threads with respect to cleaning window switches. Further, if you are interested in replacing the switch, typically these switches are not terribly expensive (mid $20's for the window switches).

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Dan

1986 560SL (52.5k miles) sold 11/24/04

1987 560sl (55.6k miles)