The transmission in a 928 is Mercedes made with some alterations by Porsche. But most components are Mercedes.
The transmission has a gear indicator and start lockout switch assembly, It is controlled by a Bowden cable that also selects the gears.
The wiring goes through the spare wheel area and the gear indicator wires go up to the instruments cluster where the starter lockout wires end up at the relay panel causing the starter relay to remain inactive when the Transmission gear selector is no in "P" or "N".
part number of the switch assembly is Mercedes 0005455106 or 0005456606