I had the same symptoms on my '83 300D. Turns out I also did not have any back-up lights. This led me to believe that perhaps a fuse was blown. It was not. But, upon closer inspection I could see what appeared to be an oily film on most fuses. So I got out my can of electrical cleaner, and Presto! All is well in tach-land. I hope this helps!

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'83 300D-T