reverse/ back up light troubleshooting

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(05-28-2018 )alborz Wrote:  stopped at the scale by DOT and complete level one inspection.

back uplight was not working.
I tested the wiring and at the reverse switch at the top of transmission checked the wires. one light up the tester but I think doesn't have enough voltage to light up a bulb .
didn't find a fuse
any help will be appreciated
2010 peterbilt 387
13 Spd eaton trans

Back up switch is NO normally open switch. When you put in R it will close relay circuit to the ground and relay will send +12 v to backup light. LCM could be used too.
To check switch - in neutral it should be some voltage present on switch terminal if wire connected or infinity resistance with wire disconnected
In R position 0 volts or 0 ohms accordingly.