That circuit is not ground dependant.
The switch itself may not be completely plugged in . There is a lock tab that has to be turned for a good connection when repalcing the switch. Just a possible.
There is a 4 prong connector around the gas pedal area with the circuit from the switch. If you either unplug this and see if you get an ohm reading [ continuity] from 1 and 2 when in R , this will tell if it is from the connector to the switch...or/if no meter, simply jumper 1 an 2 at the connector and BU lamps should light...if so, it is from the connector to the sw open circuit...if no lamps with jumper . it is from the switch back [ ie., power/lamps /wiring]

Could even be a bad switch...remember that the switch contacts are independant for BU and starter lock-out...so one can be OK with the other set not..[ 2 switches in one case]
Best bet is testing at the connector on driver side..