[caroltasha]

hi can anyone tell me where to locate the relay switch for heater plugs on a merc c250d 1994 I am having problems cold starting

[MrDanno]

Have you checked for Voltage at the glow plugs when the engine is cold? The Glow plug control unit has it's own engine temp sensor which could become faulty and the glow plug control unit is under the washer bottle on the right of the engine.

If a glow plug fails short circuit it will blow the fuse and none of the glow plugs will be working after that.