Off your engine harness you have a group of short "pig tail" wires. Purple, black, gray, brown/w/ white stripe, and tan/w/blue stripe. Purple is ignition feed, black is ground, GRAY is tach sender feed. Want your tach to work? Terminate gray to the "S" position on your tach, purple to the "I" position, black to the "G" position.
The speedo needs no power, just hot and ground (blue and black) to provide backlighting...