PEH,

You are correct. The corrosion won't cause a short but the moisture that caused the corrosion can. If you have a leaky lens the moisture tends to cling to the corrosion and can cause a short. It may well be that that is not Janko's problem but it is both easier to start there than at the gauges and also, in my opinion, a higher probability of being the faulty circuit, hense my recommendation that he start there.
LRG

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold