OK, a quick lesson in how VAG (Volkswagen, A.G.) part numbers work for those who have asked. I am by no means the top expert on this, but I will try to share what I know.
The VAG system uses two seperate systems, one for most all the normal parts, and another for many fasteners, hardware, o-rings, bulbs, etc. I will be discussing the former system that most people would want to be familiar with in dealings with the cars.
The number consists of 9 digits, 3 'sets' of 3, and sometimes a letter (or a couple letters) suffix at the end. Looks something like this: 010 321 371 B. There is a method to the madness, and here is how it works!
The first set denotes what that part fits. This set will typically be one of three types: platform, engine, or transmission. Keep in mind, many early cars (going WAY back here) will use the same set to denote all 3 types. But with all the later stuff, it will be broken down. Now, what do I mean by these 3 types? 'Platform' refers to a specific body or chassis type. 'Engine' is a specific engine that may be used in several platforms. 'Transmission' once again may be used in several platforms. This 3 digit set will tell us what the part fits, and every platform, engine, and transmission will have its own unique code. For instance, if you have a 1998 Jetta, the most common platform codes for that car will be 1H0 or 1HM. If your 1998 Jetta is a TDI and uses the AHU engine, its engine parts will mostly have a 028 code, if those parts are specific to the AHU engine. Its manual transmission will carry a code of 02A.
So if we see a part number that is 1HM xxx xxx x we can assume that whatever the part is, it fits an A3 platform Golf, Jetta, or Cabrio, and it may fit other platforms if they employ the same part.
That first set can be more specific within the platform. For instance, the Jetta is based on the Golf. They share 90% of their parts. But something Jetta specific that is NOT the same as the Golf may have a different 3rd digit. Same goes for different body types within a platform (2 dr versus 4 dr, for instance).
The second set of numbers is the main group. This actually narrows down what the part actually does. The main groups are divided into 9 categories:
1: engine and clutch
2: fuel tank, pipes, exhaust system, heater
4: front axle, differential, and steering
5: rear axle
6: wheels and brakes
7: hand and foot controls, safety covers
8: body and sheetmetal
9: electrical components
The first digit of the second set will denote which main group category the part is from. So, if our part number is xxx 1xx xxx x we know it is an engine or clutch part. This center set will break down even further to get you closer to what the part does. For instance, a '98' denotes a kit of some kind. '21' denotes water cooling parts. So, a part number that has a 198 in its center set is a kit for an engine, like a gasket kit for example. A part number with 121 in its center set is most likely a coolant hose, pipe, a water pump, radiator, etc.
The last set is the part control number, just an actual sequence number and is usually not too important in actually identifying what the part is and what it fits. It just narrows it down the rest of the way to get exact. One useful hint here: if the part in question has a left and a right, like tail lamp lenses, for instance, the RIGHT side will end in an even digit, and the LEFT side will end in an odd digit.
The last little bit is the suffix. These are not always present. Usually this denotes a newer or updated version, but not always. Important note here is that if a VAG part number ends with an X, that denotes that it is a remanufactured part, and will need to be returned for a core.
Now, all the first set codes can be tough to remember, BUT thank goodness Volkswagen has always been very good about putting the part numbers on just about every part in the entire car! This can help you quickly and easily get a replacement. Keep in mind, however, that there are always numerous supercessions as well as omissions, so these are not always 100% reliable.
Ok, so let's say you had some freak rock come flying up and break your brand new Jetta's left outer tail lamp... the part number is 1K5 945 095 J.
1K5 is the A5 Jetta platform set
945 we learned the 9 is electrical, and 45 is the tail lighting code
095 tells is which specific part, and that odd number 5 insures us it is the left side.
J just means the current revision for the US spec Jetta tail lamps
So, when you are tinkering with your car, and you see those numbers on those parts, try it out and see if you can understand how that part got its number! Also, this is very useful when ordering parts, and insuring you get the correct parts.