Good thought. If I can swap the known good relay from my 87 300D with the questionable relay from the 91 350SD that would tell for sure if the relay is bad.

Problem resolved. Of all the dumb stupid I've done this is one of them. When replacing the intake manifold there is a wiring bracket attached to one of the bolts arount the #5 glow plug. Seems the bracket was installed wrong and was making contact with the golw plug shorting everything out. Once adjusted the light comes on and everything is working fine. Thanks to everyone for your help and helping me to have a better understanding of the glow plug system.

Dont feel bad I just did the same thing on my 95 s350

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1962 220S
1966 200 Diesel
1981 300TD Wagon
1984 500SL
1989 560SEC Euro