Sorry to hear about your bad experiences w/ MB dealers, I know exactly how you feel because I have to deal with a VW dealer - talk about poor customer service! At least at the MB dealer they schmooze you nicely while they screw up your car!
Think of the free time you'll have instead of having to go back and re-do that tough job you just did! And you have the satisfaction of making MB, in effect, pay for their mistake (the defective part) by costing them the $300 labor they will incur!
Don't know about the warranty; if you ask while at the dealer let me know, I'm curious.