This seems to get asked a fair bit on here so here is a how-to.
I run a B5.5 TDI Estate with an AWX PD Engine, I've fitted an Oil Pressure, Oil Temperature and a Turbo Boost gauge and may as in my old MK4 Golf fit a Voltmeter as well.
Choice of gauges;
There are lots to choose from but I decided on fitting VDO gauges as they are OEM looking (in fact they were fitted OEM on previous generation VW's and Audi's). You can get them from all manner of places, such as;
Merlin motorsport
Awesome GTI
42 Draft Designs (and / or their UK agents)
Here are the gauges as installed in the Pasty...
And here is the Voltmeter as fitted to my MK4 Golf (coming to the Pasty as well as the W8 light when I can be bothered). The Passat is virtually the same in this area so it'd be an easy install. If you are doing this run an ignition switched live for the voltmeter and a lighting feed for the gauge and the W8 light if your going to fit one.
Below is where I chose to tap into the inlet tract (boosted side) for the boost gauge. Merlin and others can supply you with the barb for doing this. Also you may want to install a New ~South or similar anti-flutter / damper device to stop the boost gauge buzzing (yes really, they do buzz unless damped). I used an old style in-line fuel filter which works beautifully.
Another showing the boost line coming from the bulkhead. The internal route of the pipe ran inside and out through a hole in the ECU housing box.
Piccy below showing the type of olde style fuel filter I used as my anti-buzz damper
Oil pressure sender;
I used a 5 bar gauge and sender which is suitable for the TDI and mounted it here with a T piece. The stock sender for the dash oil light and the gauge sender (the bulbous thing) share this pick up. Wise people would use a twin output sender which does both jobs.
Pic below, oil senders from another angle (it is tight in there to get a decent piccy)
That is it really, the oil temp sender is simply a sender sump plug. You remove your stock sump plug and replace it with the duel purpose sump plug / sender. Simple. No pics as I couldn't be arsed !!
Wiring;
If you can wire in a stereo, you can wire in the gauges.
You will need to obtain / do;
1, An illumination feed.
2, An ignition switched live
3, An earth
4, Take sender cable out into engine bay for Oil Temp sender.
5, Take sender cable out into engine bay for Oil pressure sender.
I won't go into the detail of the wiring as if you don't understand the above a question you should ask youself is "should I be doing this?"
I'm sure someone could elaborate on the wiring side but I took ignition feed straight off the buzz-bar (think it was terminal 30), the lighting feed and earth, I used the redundant stock CD player feeds.