i worked in a dealership service department also, and agree that many (not all) techs have poor diagnostic skills. I have seen someone disassemble the frt grille area of a vehicle to find out why the headlamps weren't working. i asked if they had checked the fuses. they said yes, the headlight fuse was good. i then asked if the checked ALL the fuses, they said no, just the headlamp fuse. i then took a multimeter and tested all the fuses. turns out another one was blown, can't remember what it was labeled as, but wasn't labeled lights. replaced fuse, headlamps worked. sometimes fuses impact circuits even though they aren't labeled as such. (ex--some MB cruise cont won't work if there is a brake light out).

now i'm in the body repair side of it and see the same thing, why wouldn't you check all the bulbs in a tail light assembly before you bolt it back in and put all the trim back on? why would you disassemble the door before you checked all the fuses? i try and teach all my techs, "The simplest answer is usually the correct answer". Always start with the smallest things first.

in your situation, maybe the service advisor didn't know his regulator from a relay and was just repeating what the tech told him. most advisors are hired for their sales ability and people skills rather than technical vehicle knowledge. i know dealerships are expensive, and there are some bad ones out there. there are some good ones too. and some very good techs also. but, dealerships are expensive because good techs are expensive and take years of experience and training to make. unfortunately, there aren't enough good techs to go around and most dealerships have a couple of really good techs and several parts changers. if a parts changer gets a hold of a complicated problem, this can lead to situations you describe.

i always ask questions, i would ask why the glass brand would make a difference. would the regulator know if it was raising a piece of plywood if it was the correct dimensions? if they couldn't give me a satisfactory answer, i would ask them to look at the problem a little further, or ask for the service manager. unfortunately, situations like yours are what make people distrustful of dealerships.

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"the simplest answer is usually the correct answer"

1991 190e, 2.6 (helga)
91,000 mi