K R
Usually in the air intake housing, either the tubing or the air box. Mass airflow sensors constantly measure the amount of air entering the intake manifold. This lets the engine management computer know how much gasoline to inject into the cylinders for proper combustion. Hot-wire-type sensors, like you have, work by keeping a short piece of wire or film at a constant, elevated temperature. An electrical current is applied to the wire to maintain its temperature; the amount of current necessary to maintain the temperature is proportional to the mass of cool air flowing past it. Sounds complicated, but there are no moving parts and the response rate is very fast. Eventually, dirt and oily film bake onto the wire, insulating it from the cooler air--and when the engine management computer sees a value for air mass that's too far out of range, it sets an error code. Periodically, there's a cleaning cycle during which the wire is heated to a very high temperature right after engine shutdown. This burns off deposits. When this cleaning cycle fails to remove all of the contaminants, you're supposed to replace the MAF sensor. According to almost every manufacturer, it's not recommended or even possible to clean the delicate wires. Of course, they're selling those sensors, so their motivation might be suspect. Next time, I'd try cleaning it first--what do you have to lose? I used to use aerosol carb cleaner, but CRC MAF Sensor Cleaner is new and less prone to dissolving other components in the MAF sensor. Do yourself a favor and remove the MAF sensor from the vehicle before attempting to clean it. Also, I'd make sure the air filter housing, air filter and all associated ducting are intact, clean inside and not leaking dirt. This will keep you from needing to clean or replace the MAF sensor anytime soon.

David
It is by the air filter box. It is black and sticks out of the plastic intake ducting. I suggest you take it to someone who is competent in mechanics. The MAF is very sensitive and can easily be broken if not handle correctly. The fact that it can cost a few hundred is another reason. There is a recall from VW on this part if your mileage is under 70k.

luisitogonzalezcrazy
Air filter intake. Two or three screws. Unclasp MAF from pig tail and remove. install==MAF and plug