kontrolka is the on/off icon in your dashbord telling you the heater plugs have done their work and you can start cranking the engine; it is coupled to the control system but is not THE control system;
Re plugs: I believe heater plugs started replacing the older glow plugs ca 1980 - both are supposed to heat the combustion chamber pre-ignition but glow plugs were slow and heater plugs fast (30-40 vs. 4-6 seconds)
[Had one of them 1972 diesel Mercedeses once, suffered with them glow plugs]

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Note added at 2002-07-15 11:48:16 (GMT)
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zatem ergo jest takie: lampka kontrolna świec żarowych

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Note added at 2002-07-15 14:18:41 (GMT)
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There is a glow plug light on the instrument cluster. Note that this light goes out about 7s before power is actually cut. http://www.cs.rochester.edu/u/jag/vw/engine/glowplugs/glowpl...
or: glow plug indicator lamp