I had the same setup you describe added to my 86 245DL. Have the repaired small clock in the parts stash, it got replaced by another meter.

May I suggest you use the ground in the center of the console for the clock and the Volt meter. That is a much better ground for the Voltmeter and if you plan to add more gauges you will find that more stable noise wise.

Pull a hot wire from the switched power, do not remember the fuse number right now and run it to the Voltmeter input. You can tie onto the back of the tach for the Constant Clock power and the lights for both gauges. Blue/Red is constant power, Brown is the dimmer light power, and black is the ground. You can use that ground for the lights on both gauges. Leave a service loop (enough wire to pull out the gauge without pulling wires off) so you can change a bulb without taking it all loose.

The clock was replaced by 5 Bar oil pressure gauge, be careful once the gauge bug bites it will have you looking at ebay and egauges for more.

Good Luck,

Paul