If you're testing a circuit in-place, the plug has to remain connected to the switch that you're testing. Obviously this means you can't simply probe the plug that connects to the switch because it won't be powered. You'll have to examine the plug and find out if you can probe through the back of the plug or otherwise probe the pin you're trying to test. You may have to remove the switch mechanism from the lock mechanism to get enough clearance to do what you want.

An alternative approach is to find where power comes into your switch and where the 15x terminal is. Set your multimeter to the Rx1 ohms scale and read across those pins. Turn the switch to "RUN" and you should read a dead-short (0 ohms). If you're open or have resistance the contacts are no good.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]