Hello and welcome to JustAnswer!
An automatic transmission has a range sensor. It's signal is monitored by the multiplex integrated control unit (MICU), which is in the fuse box in the dash. The logic in the MICU is actually what powers the reverse lamps.
Since you no longer have a range sensor or automatic trans what you'll need to do is hardwire the backup lamp switch. One wire to the switch will need to be wired to a source that is switched with the key and you'll want to install a 10 amp fuse inline between the source and the reverse lamp switch. Don't use any of the existing wires from the automatic trans for this.
The second wire from the switch will need to be wired to the green wire which goes from the MICU to the rear of the car. You'll need to cut the wire so the MICU is no longer in the circuit, connect the wire from the reverse lamp switch to the green wire.
This will give you reverse lamps that work only with the key on.
Thanks, XXXXX XXXXX the MT MICU on hand and a MT gauge set on the way. I didn't know if the MICU's were different or not.
If I swap the MICU and replace the gauge cluster (which has transmission range logic as well for the AT) I wonder if I would need to rewire?
I know it will solve other issues, so i will give it a shot and if not then I will rewire manually.
Here's to repining and rewiring ah my favorite work :)