The most common complaint by VW owners is heating system problems.  To solve heating system problems, I suggest doing the following.  First, using a good set of original OEM heater boxes will give you the best heating because the internal heat exchanger has more fins for dissipating heat.  Make sure there is no obstructions to air flow from the fan located in the rear of the fan shroud on the engine.  Check the system from the heat channel up to the defroster.  Mice nests and collapsed hoses can cause poor performance from your heating system.  Make sure all cables and levers are connected properly and all heat valves are opening and closing.    I also feel that compression ratio and the type of fuel used also can affect the temperature of the exhaust that heats the heat exchanger.  Back pressure in your exhaust will hold the heat in your exchanger longer and allow it to heat up quicker.  A high performance exhaust will make less heat because heat is removed faster from your exhaust.  Proper timing of the engine and making sure all cylinders are firing will also have an effect on heating.  Remember one dead cylinder is like having 25% less heat.  Therefore, it is important the engine be in top shape.  One last thing that can be overlooked is air leaks in the body and cabin of your car.  Missing grommets and rubber seals will allow warm air to leak out.  When your car is in motion holes or cracks will act like a vacuum as air rushes past the outside of your car sucking warm air out.  Your can maximize your heating potential by following these important tips.  Happy motoring and stay warm.

Greg Curello
Rusty Bug Restorations

Other tips:
Crack a window open a bit when running the heater. If they've done the correct things and sealed up the car and got the heaters back to factory specs then the thing is so airtight that there will be reduced airflow.

Cracking a vent window can often make the defrost work much better in winter time too.