Switch on the manual trans is just open or closed, it has 2 wires.
1 wire on the switch will have 12volts when key is on.
Other wire will have 12volts only when trans is in reverse, switch closed, this is the wire that goes to reverse lights.
reverse lights share a common Ground
12volts to reverse lights is also on a trailer connector if you have one, these can short out the reverse lights and tail light if they corrode.