I don't think its a dome light switch, though that's what a lot of people think it is. It's the ambient air temperature sensor and part of the emissions system, only found on late '71 and all '72 models, and maybe in '73 (not sure). I've got it on my 72 wagon. Check out the threads below.
http://classicbroncos.com/forums/showthread.php?t=66645
http://classicbroncos.com/forums/showthread.php?t=41124