After trying to access the switch without dropping the tranny, I decided to install a switch in one of the spare spots on the center dash. Seems B.C had the same idea. I used a spare defrost switch as it has a light with it.

After a few years, I've finally got the back-ups working like they're supposed to as I recently switched out the tranny.
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1980 245 Canadian B21A with SU carb and M46 trans in Brampton, Ont.

Be glad they work at all. The switch is located on the side of the trans, up high near the shifter. It's extremely difficult to replace...and I did it on a lift. We ended up using a wrench that had been cut down and modified for some other job. The problem is that the switch is too tall for a shallow socket, and there's not enough room for a deep socket. If they don't come on while driving, then I say just deal with the minor inconvenience.
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forums.turbobricks.com

The switch is tripped by something in the gearbox, I think. When that happened on my 1983 245GL, I put a toggle switch into one of the unused panels on the control panel.

I added a small light in the same panel to indicate the back ups were on, and located the switch so that it could be flipped while gripping the shifter knob. Worked fine for years.

BUT - with the problem you have, you can mess up the mind of a tail-gater. If they get too #%@*& close, get into neutral but turn on the back-ups. Some tail-gater expressions were priceless. Better response, though, if done after dark.

Good luck,

Bob

:>)