glow plugs not getting any juice

Has anyone the answer to this problem?
My 1983 Mercedes Benz 300 turbodiesel isn't getting any juice past the pre-glow relay. I just replaced the relay and still no current to the glow
plugs. When I unplug the cable to the glow plugs from the relay, then the glow plug light indicator in the car lights up. But if I plug the cable back into the relay, the glow plug light indicator in the dash doesn't light up?
Any help would be appreciated.
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