Can you remove all of the glow plugs without anything in the way. Looks like the oil filter housing may be in the way of one.

1984 300d Turbo
Pull Cover On Relay Box And Pull 5 Pin Plug Check Ea Pin For 1 Ohm
Or Less If Open Replace Glow Plug Get Plugs At Autozone Use Deep
Socket To Remove Plugs And Hold Wire To Take Off Elect Nut, 2 Hrs All 5, Diy Done

The oil filter housing makes things tight, but it is quite possible. On the '83 126 here, there's a hose between the engine and the oil filter housing that can be moved out of the way without disconnecting.

You'll have to break the plugs free with a combination wrench, as you won't be able to get to most of them with a socket. They'll be in there pretty tight, so be careful not to bust your knuckles. Use anti-sieze on the new plug threads.

The hardest part of the job is getting the small nuts off and back on without dropping one. I recommend putting some rags under the plugs to keep the nuts on top when you do drop one.