Having replaced the ignition switch on my '86 765T just last March, I should remember more about the process than I do but the mind often purges unpleasant memories. Anyway, here's how I recall doing it from underneath the dash:

Place at least one of your favorite beers in the reefer to cool for the celebration afterwards.
Disconnect negative battery cable.
Move driver's seat all the way back and recline the seat back as much as possible.
Remove driver's side plastic panel to expose underside of dash.
I bought a $10 LIGHTED, regular length screwdriver with interchangeable bits at the local auto parts store so I could see the screws. It worked very well, better than trying to hold a small flashlight in your teeth!
I then crawled in through the rear door and lowered myself on my back to the floor. My head was jammed up above the gas pedal, my face right up next to the switch, my legs resting on the center console with my feet,pointing between the seats to the rear.
I unbolted the radio power amp from the dash frame to gain straight-line access to one of the screws. It just rested on the floor next to my head, hanging by the wire cluster.
Then, it's a matter of pulling off the wiring from the switch (it's just a friction fit) to expose the two screws.
Also, I had to move another wire bundle out of the way to see and get to the second screw.
It's very tight quarters but from then on, pretty straightforward. As your other responders have said, the switch pops off and on pretty easily once the screws are loose.
I bought and pulled a used switch out of a wrecked '85 760 for $10. The local import parts store wanted about $68 for a new one.

Good luck!

Doug