the key lock tumbler only turns a gear that turns a knob inside the ignition harness. the not turning back to lock is another part next tot he tumbler. or a busted wire in the console to the shiftier.

id start with a new ignition harness, if you got heater fan issues, parking light issues, or any other funky crap going on, start there for sure. its a pretty easy swap. take ya 30 minutes tops.

Sparky's Answers - 2002 Pontiac Grand Prix, Changing The Ignition Switch