There has to be a connection point under the dash/behind the center console panel that connect a couple of interior light related wires...did you have something connected to this (like the wire for illuminating the newly installed switch) or can the connection be released in any other way?
Normally this is a connection with a couple of (gray/blue) wires bolted to one isolated point...

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)