Any time I don't get satisfaction or feel I'm being fobbed off I just go above that person i.e. manager to area manager.
If it is a big main dealer try and find out who their Managing Director is and then write a letter of complaint to them ccing a copy to the actual manager that you have had no satisfaction with. You'll be amazed how quickly they get off their fat arses and get the job done properly.
Many moons ago I bought a Vectra Estate (1st ever brand new car). It had a squeaking noise from the rear of the car that would drive me crazy. It really was infuriating and everyone commented on it. It also had rattles from the glove box and the door frame. It was in and out the dealers several times with no success. I think about 7 or 8 full days in total. I got to the point I actually didn't want to drive it. Anyway after several attempts to fix it I felt the manager wanted nothing more to do with me and would never be available when I called or was taking the car in. I eventually wrote to Arnold of Arnold Clark fame (we all make mistakes) ccing the manager. 2 days later the manager called me personally to arrange someone to come and pick up my car and deposit a courtesy car of better spec (he gave me his own SRi estate) He also paid for all my previous insurance cover when I was using the courtesy car, upgraded my cassette player to a CD player which was a big thing then
and threw in the first service free.
They managed to fix the 2 rattles but left me with the squeak which I managed to fix by applying a tiny bit of vaseline around the tailgate seal.
So I learned it always pays to go above the bosses head if you are getting no satisfaction. Don't let them fob you off.