A lot of the solder joints are going bad on this vintage. Take a close look at your solder joints on the speedometers circuit board (magnifying glass or microscope works best). If they look cracked or there is a hairline "circle" in the joint it might be worth it to reflow/resolder the joints. If you don't feel comfortable with soldering do some googling on that. I did all the exposed joints and it didn't work so i had to remove some of the expoxy that protects the joints to access them as well. I could see through the epoxy at one joint especially was cracked so i only removed the expoxy around that one. I used a heat gun to soften the expoxy, then soldered. Put it all back and working great!

I've done the same thing with the air conditioning button on/of switch.