I had same issue till today. Access to back up switch is from right side. A month ago I had major transmission work and I really did not pay attention tile few nights back that my buck-up lights do not operate. Close inspection of all fuses and bulbs tested OK. While inspecting buck-up light switch today I found out that connector from Buck-up switch is not connected and actually was dangling in transmission
Some idiot during assembly did not attached connector.
I would suggest check all connectors and buck-up switch for contacts first, if that does not work pull switch completely and test the switch before you replace it.
I hope this helps.