On my mk2, we changed from the dg250 to the 700r transmission, so the starter/backup light switch doesn’t work right.
do I need a switch from a 420 or can we make the original switch work?

 

I think the 700r has its neutral safety / reverse light switch on the side of the transmission, if yes, you might need to extend the wire harness to it. The 420 has a different switch and shifter mechanism than the MK-2 or S type. I found this out recently.