Hi Shane,

When you remove or unmount the big in dash head clock and replace it with the big in dash head tacho, you may see as you slide the tacho in, there exist pins on the PC board the big clock and tacho slid onto. These pins provide the +12 VDC and -0 VDC ground connection through the dash head PC board. The perennial white and red wire (white with red trace) comes from the -0 VDC connector on the ignition coil.

See http://www.volvowiringdiagrams.com/

You connect that white wire with red trace to a single connector at the rear of the tacho housing. If there exist two connectors not relate to the clock power three prong connector set, I'm not sure.

At each connector should be a number and perhaps some polarity indicator as a + for +12 VDC or a negative minus (-) sign or universal earth sign as:

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―――――
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Try your Blue Bentley Volvo 240 Bible for wiring diagram help, further search the brinkboard for info, and perhaps try turbobricks.

You have several single wires connecting to the rear of that dash head. Be careful in that you re-connect everything correctly. There is a yellow and red trace wire that is a heavier gauge you do not want to connect to the tach or to the cruise control two prong connector set (usually protected by a small rubber tube) at the upper left as you look at the rear of the dash, else you can damage the speedo / odometers and the vehicle speed sensor data sent from the speedo sensor on the differential, through the wire harness, to the dash head, and then, on to the (newer) Bosch Jetronic / EZK system computer modules.

Also, as you may know, the dash head has to be connected to the wire harness complete to ensure the dash head receives power and such.

Also, safest to disconnect the battery before you remove, disconnect, and reconnect the dash head to the wire harness.

Hope that helps.

The Earl of Grey Tea.
--
Sufferin' the full moon.