Wiring behind dash

You'll need to get yourself a factory workshop manual and dive into the wiring diagram.
By matching the wire colors, you'll be able to figure out what the different plugs connect to.
Typically there are corresponding places with matching wire color, but not always.
Also not all plugs are powered and meant to be plugged into something, but be sure to properly insulate anything that has (+) current, but does not get plugged into other connectors.

Gauges typically have individual female and sometimes a male spade connector.
The lumination wires are typically bullets and or spades.

They are certain wires in specific locations for the tach.
You'll need to analyze the manufactures specs to be sure what goes where and some have micro-switches that are set based on the number of cylinders for a gas engine that runs off the distributor coil.
For a diesel it's the tack wire on the alternator.