If memory is correct, there is not wiring to old large clock. It must get its power from the instrument cluster circuit board. If you have a wandering wire back there, tape it and leave it.

The 3-wire harness from the tach to the new small clock is all the clock needs. Power for the clock, ground, and power from the light dimmer for the light. The little bulb in there is likely burned out. I have found that a hard glance can make one fail it is 10+ years old.

The clock itself is another story. I have several that are non-op. From other posts on the BB I understand that a little capacitor can be replaced. The challenge is in opening the clock housing to get at it.

Good Luck,

Bob

:>)

PS: All my bricks have a tach, some factory, some by yours truly. I really enjoy having one. See how the car feels if you make it upshift at 2500 to 3000 rpm. Start in "1" and you can control the shift points. Have a lot of clear road. Feel the increase in power between 2500 and 3500 rpm. Surprise a Mustang. BTDT

Next gauge - a voltmeter, mounted below that small clock.