Won't shift out of park, left turn signal comes on when brake applied.

I bought this truck Friday. Test drive it seemed good. Drove it home. Then took it to a Ford dealership to talk about putting tow package on it. Got in truck would not shift out of park. Left it for them to fix. Sat they said they replaced starter switch, but suspected an electrical problem, because when brake pedal depressed left signal light comes on. Was able to drive truck to three places, turning truck off and on and driving. Came home, parked truck went to leave later could not shift out of park.