Yes, the problem is with the glow system. There are two possibilities...one bad glow plug can do this on a series-wired car. If you have tools, measure the resistance of each plug.

Or the glow plug fuse may be done. I'm not sure where the fuse is on your car, but on my '79 300CD it was in a small black box on the firewall. It's not in the regular fuse box, and it's not a "normal" fuse, but rather a strip of lead. I had this problem once, and not knowing what to do, called Mercedes service. They came out, shorted the fuse circuit with a crowbar and had me crank. Started up and I went home. Travelled with extra fuses after that.