Based on the document at http://www-pal.usc.edu/~jan/vw/Body/gauges
Compiled and maintained by  Josh W

Gauges are cool.   To state the obvious, they let you know what's going on with your engine, which can boost your confidence when it comes to taking a long trip or pushing the car to its limits.   Several VW/Audi variants featured 2^1/16" gauges, and they were optional on most models. If your car came ithout them, it's not that hard to add them.  

A short list of donor cars- feel free to correct me.   I'd also love to hear about what cars came with which gauges in countries other than the U.S.    "+" indicates optional on the base models.

Notes:

Oil Pressure:    If your car didn't come from the factory with an oil pressure gauge, then you'll need to get a differrent oil pressure sender. New ones cost about $60, so you might want to grab the one from your donor car.

Also, cars without "dynamic" oil pressure warning systems are a little tricky.   This would include '82 and older Rabbits and pickups (maybe others?).   Dynamic systems warn of pressure under 2 bar when RPMs are over 2k, as well as if the oil pressure drops below 1 Bar at any time.   Dynamic systems used one sender on the head, and another on the oil filter flange.   That $60 sender is geared for the dynamic system, so if you hook it up on the other, your warning light will be on all the time, because it's not getting the proper voltage.   I'm told that installing the speedo cluster and oil filter flange from a dynamic car will resolve this - I'll keep you posted.

Wiring Harnesses   If you are pulling a gauge set out of a car, you should try to pull the intact wiring harness for it as well.   They are sepperate from the main harness, and can easily be disconnected from the fuse pannel area.   If you are installing the gauges on a similar make/year, you will probably be able to reattach th harness to your car's fuse box - with no cutting or crimping!   Well worth the hassel of digging the wiring out of the donor car's carpet.

Oil Temp   The oil temp gauge connects to a sender on the oil filter flange.   Many VWs (e.g.: my '81 diesel truck) don't have senders on the flange.   You will need to replace the flange if you want to use the gauge.   Might as well install an oil cooler while you're at it.

Alternately, you can purchace an oil temp sender from VDO that replaces your oil pan drain plug for about $30.

Installation:

Oil Pressure

Attach a connector to the end of the green wire, and hook it up to the terminal marked "G" on the NEW sender. Cut the plug off the old sensor wire, and install a connector on it, attach it to the other terminal off the NEW sender.

Reinstall the headlight switch, and check all wiring, and also check for left over tools. Reinstall the (-) to the battery. Insert key to the "on" position. The gauge should set itself to 0 psi. The oil pressure light should blink as normal. Start the car. The oil pressure gauge should move, the idot light should go out. Check for leaks at the sender.>

Ammeter

Ideally you want this connected between the battery and both the loads and alternator so that you can tell whether you are charging/discharging the battery. Unfortunately, both the alternator and the main battery circuit go straight to the battery on the water-cooled cars. You would need to break the connection of both and insert some long wires to the gage. This adds a fair amount of resistance (unless you are using welding cable) which will drop the voltage available to your system under heavy loads (like A/C or headlamps). I find an ammeter to be more trouble than it's worth to install correctly and would rather have two voltmeters.

Voltmeter

The best point to connect to is right across the battery. The only problem with this is that it will drain your battery with the ignition off! Next best point is connect the poitive terminal to the switched side of the ignition key as close to the switch as possibe (this is circuit 15). Connect the negative side to the ground strap at the battery. Any size wire is alright since the gage draws very small currents. Connecting the gage across other points (like at the radio or other dash components) will give rapidly changing results as different loads come on and off and cause voltage drops throughout the system. Also this more remote points will give a lower average reading. And ideal situation would be a voltmeter connected to the switched 15 circuit at the fuse box; and another one connected directly to the battery through a relay that is energized by the 15 circuit. The battery voltage will give a good indication of charging behavior. The fuse box voltage will give a good indication of general electrical system degradation with age.

Tachometer

Positive power lead to circuit 15 (switched ignition line) at the fuse box (after a fuse - say radio), negative lead to ground, and the signal lead goes to the switched side of the ignition coil (circuit 1).