pull off the tailgate internal trim (torx 20 screws) and check the voltage at the motor connecter with a multimeter, one should have battery voltage with ignition on (possibly yellow). one should be pulsed with voltage every few seconds (with switch turned on), and the other is earth
If the pulse wire is live all the time its body module at fault. If there is only one wire live and motor is running pernanantly the park switch is sticking inside motor. replace motor
