Yes I know it's not really a toggle switch but I used that word because it toggles the solonoid and light off and on. Actually post 85 or so there is only 1 contact on the rely used for both the light and solonoid.
As I analyze the circuit it works like this:

12 volts is supplied to bulb, other side of bulb goes to relay contact, then to solonoid, when solonoid is open, bulb lights as it gets a ground through the solonoid coil.

When solonoid picks 12 volts flows to solonoid and it too picks but now there is 12V both sides of the bulb so it goes out.

This circuit is often misunderstood by Bricksters, earlier models used the NO and NC contacts on the relay to do the same thing.

It really is amazing what a flush can do for the durable AW70 trannies especially if you go to over to synthetic, that has been my experience. I have cured 2 tranny problems (hard shifting and no OD) on my old 740 and my daughters 240.
Flushed my current V90 well as a preventive measure.

David Hunter