We do parts warranties for clients who had genuine MB parts
installed at a indie all the time. As long as they have the reciept for the MB part and a reciept for having it installed at a real
shop. They then bring us the car and we proceed with our warranty work just like a new car.
BUT, if the new MB part is not failed, then the client picks up the
tab for any labor time invested.
I just warrantied out a 617 turbo cage motor installed by a indie
4 years ago for oil consumption. All the client had to buy was a set of motor mounts. Not a dime in labor.