NV4500 reverse lights

[1967KaiserM715]

Have an issue on my truck, 1985 GMC K1500, I put a NV4500 in from a 1995 GMC, reverse lights have been working fine for a while, but last 2 or 3 months I noticed they seemed to have stopped working. I check for voltage at rear bulbs(transmission in reverse, and ignition on.) Had nothing, so I disconnected at switch on transmission. Jumping pigtail allowed voltage and gave me lights. I assumed a bad switch, so I got a new one and tested before install. It worked fine uninstalled. But still nothing when in the reverse gear when new switch was installed. I'm not too familiar with how the switch and reverse gear work together.

What should I look for? (I can hold the shift lever all the way over and down and still no lights, so it seems like no switch contact at all.) Thanks.