What Up?!?! 06 Mustang GT (Auto)

Looking for ACTUAL experience on splicing a killswitch into the ignition starter wiring a little bit further down the line. Understood our cars have a "built in" need for the key but bullhonkey on that.

Don't want to splice into the under hood harnesses or wiring to focusing on locations inside the cab. Has been difficult to find understandable wiring diagrams with color coded pictures like in the 1st grade.

Thanks in advance for any and all info