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The glow plug relay is on the driver fender behind the ABS pump. It's a black box about 4" by 4" by 1.5". There's a plastic cap on the relay so you won't see the connections. The cap just pulls off though you have to navigate the tight quarters. The glow relay has a live battery connection so be careful with metal tools, or better still, disconnect the battery. There is an 80-amp strip fuse in the relay. Make sure that fuse is intact. Best to remove it and make sure it comes out in one piece. Sometimes it breaks so cleanly that it looks intact. There is a big 6-pin connector and a small 4-pin connector. The big 6-pin connector has the wires to each glow plug. The connector sockets are numbered corresponding to the engine cylinders.

Glow plugs don't have a service life per se. Replace them when they go bad or when they're accessible, not because of age, miles or number of cycles. Get Bosch glow plugs through the Buy Parts link above. Autolite don't last.

If you have to replace glow plugs and this is the first time, it makes sense to remove the intake manifold. There are 12 bolts that take a 6mm hex tip holding the manifold to the cylinder head. It takes a long hex tip with a wobble tip. First the crossover pipe has to come off. There are 2 bolts that take a 5mm hex tip between the crossover pipe and intake manifold. Put a rag under the junction to catch the gasket that might otherwise slip between the block and IP. I've yet to see a gasket that wasn't reusable. There is a further bolt and nut and bolt pair holding an angle bracket at the turbo end of the crossover pipe. Remove those as well. The horizontal bolt has a nut on the opposite side. You'll have to reach it by feel with a 10mm wrench. There is a big green o-ring holding the crossover pipe to the mixing pipe. Wiggle the crossover pipe off the o-ring. I've yet to see a big green o-ring that wasn't still round and supple even after 300K miles of service. All o-rings should be this tough. Then release the injector lines at the IP and injectors. I use a 14mm wrench but it's more appropriate to use a flare wrench or the special crow's foot socket. What's left are bits and pieces like the boost signal line, overboost pressure sensor wire and various wire clamps to remove before the intake manifold comes loose.

Now is a good time to check that the injector line clamps are in place and intact. I'll leave that for another thread

Sixto
87 300D