When I first got my '78 Bus, the heaters were poor. So I went 
     underneath with the engine running and found that where the two boxes 
     join at the metal fork pipe piece (the bit that rusts), the pipe 
     section to one of the boxes wasn't hot at all. That was some time ago. 
     Since I just rebuilt the engine I found out why only one box was 
     producing good hot air.
     
     When I removed the heater boxes and compared them, they were totally 
     different inside. One (the genuine one) has a finned aluminium cast  
     heat sink around the exhaust pipe inside. The casting fits the heater
     box tine very closely and has alot of thick fins. It therefore shifts 
     alot of heat out of the box. The other (cheaper) box uses what appears 
     to be some sort of clay or putty to form a kind of finned heat-sink. 
     Just by looking at it you can tell that the surface area of the clay 
     heatsink is much less than the aluminium plus the properties of the 
     clay one as a conductor of heat are probably poor. Not to mention the 
     quality of the exhaust pipe running through it - I don't know about 
     that, but I can guess.
     
     The *ANSWER* is simply if you intend to keep the v-dub for a long time 
     invest in a genuine part (I don't know if Brazilian ones are made like 
     German ones) not an aftermarket cheapy. 
     
     Just my experiences.
     
     
     Lee
     
     L.S.Griffiths@Physics.Salford.ac.uk