Electrical / wiring question

I just comolested an engine swap on a rear engine Snapper. Went from an 8hp Tecumseh with a bad crank to a 12hp cast iron Briggs. Everything is working correctly except for the kill switch. The engine will not cut off when the key is turned to the off position

There is a yellow wire that will kill the engine when connected to the magneto. There is a black wire from the magneto to the linkage control and there is a tab where that black wire connects that sure seemed like where the yellow wire should. connect. When it is connected it grounds the mageto regardless of the key position.

I can't follow that yellow wire back, it's buried in a factory harness that I'd rather not disturb. The yellow wire, a red wire and a black wire go in together at one end of the harness and only a black ground wire comes out of the other end of that harness.

I can touch the yellow wire to the engine block and it does not kill the engine but if I touch it to the magneto wire it kills it constantly. The easy fix would be to connect the yellow wire to the magneto wire and install a simple inline toggle switch but I want the key switch to work correctly to kill the engine.

Any help completing this wiring would be appreciated.

Disregard.....Yellow wire is part of the safety switch circuit. Kills the engine when connected to the magneto wire......unless of course you're sitting on the seat! I'm and idiot but glad it's working like it's supposed to.