Well I cleaned all the grounds but there was no effect. I then decided to add a ground and to my amazement the tachometer and oil pressure gauge came back to life. Unfortunately, as soon as I turned the turn signals or either the park or headlights on, both of the gauges went dead but would come back to life as soon as I turned them off. Where can I add a suitable ground that won't be affected by what is obviously the instrument lighting?

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1989 300CE