If you mean the shifter is locked in park, then check the sensor on the brake pedal. Also, if you just turn the key to the first position, with the engine off, you will be able to put the shifter into neutral and start the engine from there. Then just slip it into reverse or drive.It will still go into park after.
You don't mention what year of Mercury Grand Marquis but the owners manual will explain what to do if the transmission gear shift lever will not shift out of ( P / PARK ) with the brake pedal depressed ( it has what Ford calls the " brake shift interlock " ) As of August 6 , 2012 you can still view the Mercury Grand Marquis owners manual online ( back to the 1996 model ) at : www . motorcraft service . com ( no spaces ) Click on Owner Guides
Try adjusting the modulator valve
check your brake lights if they not worhing replace fuse or brake light switch
sorry that's a aod transmission they don't have modulators.the shift is controled by the kickdown cable
what will fix the overdrive light from flashing in a 1996 mercury grand marquis its a gear ratio problem between 1 and 2 shift
Into Park or out of Park?
Just turn the key to the first position, ( not on ) and then you will be able to shift to neutral and start the car from there. Have the fuses checked and check the shifter sensor on the brake pedal.
Stuck in Park? Check to see if the stop lights are working - shift interlock is the same circuit - check the stop light fuse Try to turnthe key to 1st (unlock) position, shift to neutral start vehicle and the shift to Drive
Find the switch attached to the brake pedal for the shift interlock and jump the two wires together. Be careful not to disconnect the brake light switch
If it works like the one in my '94To set it: in park press the pedal to the floor.To release it press the regular break pedal and shift out of park.
do not put any pressure on shift lever until brake is pushed. also lift shift lever directly up when placing into reverse.
Best I can tell you is that it is electronic. Check at your local Ford parts dept. They should be able to help you.