i went thru the same problem with my bro's 92 LS. It is the gear selector switch (the one that commands when the lights P, R, N, D etc to light up). right at the end towards the park position there are 2 wires that will sense that the car is in park or not (independent of the p light). sometimes the switch won't close the circuit to allow the interlock and the ignition lock switch to be activated when turning the car on or off. i used some aluminum foil as a temporary fix, but it stopped working for a few days. now i'm in a search for that switch an no one seems to carry it.
