A similar lighted mirror was used on the 850 Sedan (which the OTAS is based upon), as well as some 124 Coupes. The light was held in place within the mirror via a tension spring-clip on each end, not using 2 screws as yours has. Otherwise, the light unit looks the same.
I can mail you a decent used 850 Sedan light assembly if you want?
You might also be able to fix yours using the black switch toggle piece from a 124 Spider interior courtesy lamp (the one in the center radio console) or 850 Coupe interior courtesy lamp (in-dash above the ashtray) - the toggle might be very similar(?).