Tach Test?

[Ironhorse]

Can someone with a factory dash tach tell me how to test mine for operation before I put it in the new bezel and then in the truck? Have the tach, fact. GM printed circut, and the original harness. Heavy red wire with two clips or connectors, one has short pigtail connector and then a connector.(12 Volt power which hooks to 12 volt source unfused at fuse block), heavy brown wire (Hook to Coil Positive or Tach port on HEI) and then what? Does the tach use a ground? There is a threaded stud or attachment above the connections on the back of the tach.
'Preciate any help before I smoke test this thing. Got lucky couple years ago and found it in a truck at a 'yard for $15 for complete dash. Just been waiting until truck gets a complete re-build before I put it in. Thanks for any and all help.

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1971 Chevy 1/2T 4x4 LWB 350v-8
Wellington, Ks