7MGE automatics Have a neutral lockout switch. If you still have the old equipment look for that electrical connector and how it functions when the selector is in park. You could figure out what needs to be shorted to make the ECU happy. Otherwise, you could run a direct connection to the starter and have a button on the inside. It may crank but not start if the ECU isn't satisfied. The online Supra manual has a schematic of the electrical system.
Check it out here:
http://cygnusx1.net/Supra/Library/TS...l.aspx?S=D&P=3