Welcome. Is your car a wagon? {wagon is 300TD, sedan is 300D}
I have had several trunk lock vacuum actuators leak and cause your door lock/ignition turn off issue. It is definitely a vac leak. In the mean time, under the hood near the firewall you will see several colors of vac lines go through a rubber grommet in the firewall itself. Until you get a Mityvac and troubleshoot your leak you can unplug the plastic lines from the rubber connectors just prior to the firewall, plug the rubber connector, and see if your car shuts off.
Yellow is door lock. Yellow/gray is vacuum cannister in back. Green is climate control {usual source of leaks} brown is your shutoff and black is a vent line.
Try unplugging green and plugging connector to that {golf tee works good with that}. If still not shutting off try plugging the supply for the yellow line. You can then easily operate your door locks manually until you find the source of the leak.
If the leak is in the green line and you tee if off your door locks might work at that point if no leaks in that circuit. {yellow}
One tank wvo conversions are not usually looked upon favorably here fyi. Not my area of expertise as I give my $$ to big oil......
