The problem is most likely in the shifter assembly or the circuits and devices that control the interlock.Check the transmission by disconnecting the shift cable and manually engaging the transmission. I believe there should be a little cover next to the shifter you can pry off and stick a long screwdriver in to get the shifter out of park Once you get the console of check the park position lever to see if it is bent or out of alignment. Also make sure that your brake lights are working. If not, replace the fuse since this will also lock your shifter in park. Good luck.