Use a socket with a univeral and long extension. Works every time. I can even get the glow plugs out of a 603 without removing the intake manifold with a good universal joint.

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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair