Oi! I was going to say that your ignition switch was probably sticking in between KP2 and KP3 like my 740 did, but I guess that's not the case!

I would carefully trace the wiring from your battery to starter and make sure it's intact. Then, rather than trying to trace the wiring for the starter from the ignition, I would run a completely new wire (encased in that black wiring tubing) to the starter.
I would also double check the way the wiring attaches to your ignition switch. I don't understand how the engine could stay running if you cut its spark and fuel supplies. By the way, how did you get the engine to stop running?

I would also point out that if this wasn't happening before you did the blower motor and heater core, there is a possibility you moved some wires into a position where they started shorting out or making contact with each other in the course of the job. Does everything else work right?

How did you rule out the ignition lock cylinder?

Oh, and congratulations on completing two of the worst (longest) jobs in the book. The car is not doing a good job of being grateful.

Good Luck!
--
1990 740 Turbo, on its way to stock specs, maybe beyond