Had my car for a couple of months now. Ever since I had it, the glow plug warning light used to come on for a few minutes after I started the engine from cold. Also, when the temperature dropped to below 5c, it took a good while to start. More recently, I changed all 6 glow plugs but the light still came on. This suggested the glow plug relay unit. I got one from my local dealer for around £140. It's a revised unit. A little smaller than the old one but with the two mounting holes in the same place. On my S class, the unit is pretty accessible. Take off the engine cover and it can be seen towards the front driver's side of the engine. It's held in upside down by two torx screws to a metal tray that is bolted to other parts of the engine. Undo those torx screws and then then the two torx holding the unit in place and it wiggles out. There are two plugs which go into the unit. One kind of oblong and one round. Pop the plugs off, plug them into the new unit and then re assemble. It's a pretty easy job and was done within half an hour. Ever since then, I haven't had the light come on. Just waiting for a cold night now to confirm it will start ok.