Freightliner 1991 back up beeper need help please

Guys is me again sorry but I never had a diesel truck ever and I need to certify it, PO cut the reverse beeper wire and I can’t find where is the wire so I installed a new reverse beeper and wire it to the switch on top of the transmission where is already 2 wires connected, a white one and a red one but obviously that didn’t work since when I open the switch with truck in reverse it was working but as soon as I remove it from reverse gear it kept beeping; so in other words as soon as the switch is on start beeping all the time.
I tried to leave the red wire connected and tried to run the white wire inside with an external switch to manually so it and it didn’t work either. It was doing the same.
So I am guessing the switch is broken or that little switch on top of the transmission is for something else.
Please any info will be appreciated!

 

Guys I just make it work temporarily hooked up the positive to the white wire on top of the switch and it is working for now as it supposed to do but it is connected directly from the beeper positive to the switch on top of transmission, I think I should be ok for now until I find the end of the wire PO cut.