I crawled under my 65 and saw the old switch mounting area, but just haven't replaced it yet. I guess I should get to that replacement soon, but just haven't actually needed backup lights. It's just a luxury we really don't need, and the switch is an expensive item. Keep searching for answers from other posters, and post some pics of your Mustang.
Here is some information that may be helpful.
The toploaders have a little hole on the shifter mechanism about 1 cm diameter where a little push button switch sits - put it in reverse get under the car and have a look - from memory it will be on the front of the shifter.
It's just above where all the linkages attach to the bottom of the shifter. If you can imagine the main shifter chassis frame where everythings bolts up - its on that.
Put the shifter into reverse (inside car) it sends the lower shifter lever forward which then pushes on a little round switch - or near where this hole would be. You can always drill your own.
My toploader also has a little switch built into the side of the gearbox (2 pin plug). I thought that was the reverse switch when I first got the gearbox - but it didn't work - who knows what it was?