Mercedes-Benz of Palm Beach

I am very disappointed with the level of knowledge of the techs and the process of fixing a vehicle that comes into this facility. My service advisor was very polite and accommodating, that's about all I can say. The car was towed in for a no start (2009 Mercedes GL320), only to be told that the glow plug module is bad and that is the reason the car is not starting, now I am very mechanically inclined as I do work on cars as a hobby whenever I have time. I knew the glow plug module was bad before bringing in the car but any good mechanic should know that alone wouldn't be the reason the car is not starting as we live in South Florida and not up north where the weather is cold. I was given a repair estimate of $1400 for that glow plug module which costs $135 anywhere else and it is located in front of the engine so it would literally take someone 1 hour tops to replace it, I was also told the car computer will need to be reprogrammed once the glow plug module is replaced which I know that is not necessary or required at all once the glow plug module is replaced. So here lies the issue: I paid $338 for a diagnostic fee just so Mercedes can give me wrong information due to their knowledge of cars. When I asked if they started the vehicle to drive it around and have their diagnostic computer plugged in while the car is running (this would give more accurate numbers and information) they said no they didn't because they couldn't get the car running, which is also not true because you can bypass all sensors and start up the vehicle with starter fluid if you really wanted to, which is what I did and drove it off their parking lot, by doing that I noticed that the car was lagging and was running in limp mode, so that means the starting issue is a bigger problem then the glow plug module but the Mercedes technicians wouldn't even know that because they are not even mechanically inclined to get the car started in the first place. With all this happening I asked the Shop Forman and my service advisor what they would do in this situation, the answer I got was well the only active fault is the glow plug module malfunction, so we would need to fix that and it should start up and work, to which my follow up question to them was "what if that is not the core issue why the car is not starting, and I pay you $1400 to replace the glow plug module and then once it's replaced the car is not starting still, what happens then?" Do you come back at me with "well that doesn't seem to have fixed the issue? The next thing we need to replace is this and that and here is another repair quote of god knows what ridiculous amount that is more than the Mercedes itself is worth. So then I express all these concerns to the Service Manager Mike and he advises he would reimburse me the $338 diagnostic fee for sure he just needs to speak to his shop foreman to go over the situation and then advises he will call me back the next day and a day passes no call no nothing so I emailed him back and I get a answer "well if you would like you can bring the car in again so we can diagnose it for free" (yes let me town the car in again and spend $200 on towing after I already did that once plus paid you $338 for nothing), so basically a big [non-permissible content removed] to a loyal Mercedes owner since I was 18 years old now I am 32 by the way I am very disappointed that a brand like Mercedes would employ technicians and staff that are not best in the industry.