I just changed all 5 glow plugs and the relay on my 83 300SD. This is the second time, and I will NEVER do it again without
pulling the fuel lines or something else off. I ended up with several hours of nasty work that defied an assortment of wrenches & sockets. Also, Bosch doesn't bother to give you a new nut with the glow plugs so you're out of luck if you drop one (or more). My MZB dealer didn't have any, so I ended up at a NAPA store buying 5mm nuts at 8 in the morning along with some more sockets and extensions that didn't help any...