Service department stealing parts from cars

I booked my car in for a service... standard service with nothing else wrong with the vehicle.. when I collected my car my passenger window was lose and a day later it fell and had to be taken in ... I decided to take it to a different dealership because I had concerns about shiraz auto .... I had taken it to the glen branch .... they had accepted the car and called me to confirm that the dealership (shiraz) had opened my door panel and stole parts from my car .

I called shiraz to let them know that I am aware ... they had agreed to collect the car and fix the window and have asked me to drop all cases opened against them with Mercedes.i refused It's a month I still don't have my car back God knows if they are stealing more while they wait for the part that they have stolen from me ... I am disgusted at the way they have been operating and do not trust any1 that works at that dealership .