Since it wasn't specifically mentioned elsewhere (at least that I could find), I wanted to comment that I've just done this mod on a 2004 Jetta PD - no problems.
Holding the new tank and looking at the space it has to get into, I was a bit nervous that it wouldn't fit, that maybe something was different on the PD's, but it actually went off without any difficulty. The only casualty is an oddly shaped plastic wiring harness clip that must be trimmed to reside under the 5.5L tank (with the 3L tank, it comes up the right side).
Plan on ~3 hours to be safe; that's probably about what it took me (I did the coolant heater at the same time, so not exactly sure), and this was the first time I'd worked on the Jetta. If you already know how to get the bumper and such all off, you'll be faster. Especially if you're not working in below freezing weather...
I could probably do it again in half that.
My instruction notes:
- Drain your 3L tank! Turkey baster works great!
- Remove engine cover (pull straight up at the front left and then front right corners, then work it over the donut at the rear centre)
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[Edit: People have reported that it is not necessary to remove the bumper in order to do this install, at least on the Jetta (post 1, post2). Probably makes things easier to remove it, but adds considerable time to the project.]
- Remove grill (just clips on, a bit tricky to remove the hood release latch - flip up the metal clip and pry open, over the pins)
- Remove left and right lower grills
- Remove bumper clip (4 screws in each wheel well, ~8 screws across bottom, 2 bolts behind left and right lower grills, and the bolts that were behind the grill - 3 or 5, I can't remember)
- Pull bumper gently forward enough to rest on the ground so that you can unplug the turn signal connectors - they just pull straight out.
- Disconnect headlamp (a real PITA - pry it off with a screw driver from the top, it really is the only way).
- Remove headlamp (2 screws on top, 2 on bottom)
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- Disconnect fuel filter (2 bolts through the sheetmetal, then lever forward to clear the sheetmetal and pull straight up - this just gives you some room to work)
- Remove 3L washer tank nuts - one beside fuel filter, one beside intercooler (you'll need a long extension with a 10mm socket)
- Unplug heater plug from 3L tank (just pull off).
- Disconnect fluid pump - pry out (away from tank), then pull up - it's just a 1" nozzle that pokes through a rubber grommet.
- Remove 3L tank (work the tank off the top mounting bolt, then the bottom - you may need to gently pry the nearby hose to get it off)
- Remove air conduit (pull the top and side hoses from their moulded guides, and the wiring harness that runs just inside it; remove the "C" clip at top (MAF) and bottom (intercooler) - the hose pulls straight out of it's mate; you should be able to remove it completely with a bit of twisting and turning - just be mindful of nearby harness and connectors. Check out how much nasty oil is in that thing!
- Transfer all grommets and plugs to the new 5.5L tank (don't forget the donut for the bottom!); for the heater plug (if so equipped...), pry the *plastic* part up with a screwdriver and it will pop out - be careful not to damage the grommet.
- Remove the wiring harness clipped more or less below where the tank sits (pull straight up) - there is a vertical moulded piece of plastic that must be cut off. Remove the electrical tape and make it such that the harness will lie flat; make sure there are no sharp edges. Reinstall.
- Take a moment to reassure yourself, while holding the new tank and looking at the small hole it needs to get into, that IT WILL FIT!
- Disconnect the small connector beside the radiator and remove it from it's holder - this will completely free the harness, and give some wiggle room.
- Angle the tank in, aiming for the fender. Before getting too far, install the fluid pump which is now on the back side (you may want to slightly reroute the outlet line - it just pulls off) - angle the nozzle into the grommet and insert all the way down, then push the pump towards the tank until it snaps in.
- At this point, just work away at the tank until it is installed. Pay attention to the wiring harness, making sure it's not twisted or caught up. It's really not too bad getting the tank in.
- That's about it - install the tank bolts, connect the heater plug, install the fuel filter bolts, install the intercooler bolts, install the air box conduit and reseat the hoses and harnesses in it, reconnect the other harness plugs, install and connect the headlight, install the bumper and grill (don't forget the indicator connections). Move the tank cap from the old tank to the new (pry it off, and note that it is "keyed" to the new tank with a slot). Install some washer fluid (don't fill it just yet, in case there happens to be a leak), and test it all out.
A worthwhile upgrade, IMO, especially if you routinely drive in salty slushy roadspray mecca's like Ottawa. Tedious, but not overly difficult. Just make sure you have a long (12") socket extension for some of the hard-to-reach bolts.
Cheers,
fitzski
when I tried to refill a w/w reservoir in any car, just to find that I can't get that last 1/2 litre in. Given our salt-spray winters I should do this mod right after the TDIHeater I'll be installing on the weekend (in the snow, if things continue as they have today 
