Mike, something doesn't make sense here. Shaun and Achim are surely correct in concluding that the switch is an improvisation; beyond the fact that they both know their stuff, the part number is nowhere near an M-B number, much less one contemporary with our cars. And plugging something in at that point would be a tidy bypass/shunt/jumper - - except that you seem certain that the switch is a normally-open momentary type, which couldn't possibly serve in that capacity. So just to be clear: you've tested the switch and are certain of its function? [If it were actually the opposite -- normally closed -- of course that would make it a workable fix, and I have seen some that were convertible though that one doesn't look like it.]
Certainly is a puzzle as to why such a thing was chosen, and even more, how your dash lights could function at all with the circuit normally-open at that point. Maybe it's a problem with terminology: electricity-speak for a switch that passes current through is to refer to it as closed, and an open anything -- switch or entire circuit -- will not allow current through because there is an open gap there, so no path for the current to follow. It's like taking out a section of railroad track or blowing a bridge, not like shutting a valve.
We have excellent circuit diagrams in our Tech Manual section which are accessible to full members. [Most of the TM is open to all but some of the best stuff requires joining up.]
Welcome. And when you get around to it, being a full member costs something on the order of 10 cents a day; absolutely the best bargain I know of.