Does you shifter have a rocker switch marked W S ?

If so, it might have a cable running from the ignition switch to brake peddle then to the left rear of trans. ( The R 129 with 722,6 electronic shift trans has this )

When you push on the brake peddle with the key turned, it pulls a cable than unlocks the shifter at the trans. There is also an interlock that prevents the key from turning fully off if you are pressing the brake peddle.

If you don't have the W S rocker, there still should be some sort of cable running from the brake peddle.

The electrical solenoid is a reverse lock out.