Can a repair should deny liability due to a faulty part in the car?

I recently had my oil changed at a local mechanic. After he had finished the oil change, the mechanic reached in from outside the car and turned the key, the car was in gear and it drove off the lift and into the wall. The mechanic is denying liability, claiming that the car only started because the clutch interlock switch was faulty. After I had the damage repaired, I found that the switch was stuck, and it has been replaced.

Even with the faulty part, can I still claim that the mechanic showed negligence because; 1. the car was left in gear on the lift, 2. the hand break was left off, and 3. the mechanic reached in from outside the car to turn the key and turned it too far.

Even with the issue with the interlock switch, there would have been no damage to the car if the mechanic had applied the break, or was in the car when he turned the key.