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

:>)