Not sure you will be able to locate a NOS / OEM replacement. Aftermarket ones are sold on eBAY and you will have to check and make sure the in line fuse is included in the harness along with at least minimum length of harness to connect. It is possible to disassemble the Mercury Switch from the Housing while housing still attached to vehicle. There is a Snap Ring like retainer right behind the Housing that holds the Bulb/Mercury Switch in place. Remove Snap Ring retainer/Mercury Switch and it's plastic housing will slid back and out, allowing you to inspect/replace. If taken apart, make note of pieces during disassemble so that you can reassemble correctly, if needed. I usually have to take pic's................
I also have a 72 Monte Custom. I have been keeping a Spreadsheet on those 72 Customs still serviceable in order to get a idea of just how many are still roadable. Unlike the 70/71 SS, GM never distinguished with production numbers the difference between the Base Model Production and Custom, there is NO WAY to determine just how many were produced. That Spreadsheet is located on this Site under 454 SS thread, The 1st Article 72 Custom Numbers? has the most up to date info for the Custom and is the Thread I started. Feel free to look it over and then decide if I can ADD your Vehicle to my Database. You will see the pertinent info I included along with YOUR VIN# that helps prevent me from duplicating them by cross referencing the VIN's. If you have a Build Sheet, include a pic of it and if legible, can copy the info from it. Otherwise, you would need to copy those Options for me.
Let me know if this is possible. I will say anyone can view this Spreadsheet and you can also make a copy for your own use. Only active Members can make a copy...........
Doug