The switch is in the oval can at the extreme top rear of the gearbox that covers the three selector rods. It's not on the cover. It's held in by two screws going in from the rear, and you may be able to reach up into the tunnel and access them.

There's a single terminal on the can connected to nothing but a little spring. When the reverse selector rod moves back, it's supposed to contact the spring and provide ground to the terminal, and therefore to the relay.