My rear switch on my fxrs went out some years back. I checked with HD for replacement....$$$$. So I went to Pep Boys, bought the cheapest one they had (for MUCH less than HD) and it worked fine. Didn't even have to bleed the brakes! I don't remember anymore, but I think I had to make some small modification to mount it, but it was simple.
BTW, if you have an older single sealed headlight like the fxr, you can save big bucks on replacement by using the low/high lamp made for the cars with the dual headlights. It fits perfectly on the fxr and because it's the low beam half of the duals, it has low and high built in. The high lamp is high beam only. Also, get the right side light, as it's aimed a bit to the left.