this guide is for owners of the car, diesels only, models from 95 were fitted with a booster heater, this is a unit fitted underneath the car under where the cenre row passenger sits on the nearside.  its a cylinder like object with a small exhaust pipe coming from it. it also has 2 water pipes going in and out . its purpose is to assist the engine in producing hot water more quickly in cold weather. you know if its working as it gives of a distinct humming sound and sometimes exhaust ssmoke   also it runs on after the engine has been stopped sometimes. its supplied with fuel via a t into the main fuel line. if your car seems to take an age to even start to warm up chances are its faulty.  the usual cause is a duff glowplug. this sits in the unit and the unit has to be lowered from the car to do,  you can unbolt the bracket and lower the unit with water pipes still on    you must then remove the torx drive bolts to seperate the unit into 2. the glowplug can then be removed for checking and replacement.    sometimes you need to take the unit to a workbench   in which case the water pipes need clamping with something .  the torx bolts tend to rust and need to be drilled out   have new ones ready available from vw dealers, dont bother with seat or ford as they may tell you they are not available. on re assembly hopefully the unit will fire    or give it a gentle thump with a hammer   the fan may have got stuck. there is a way to test yours   by bridging a sensor.    in the engine bay on the n/s wiper arm linkage  there is a small round cylinder shape sensor   remove this and pop in across the terminals a 10 amp mini fuse   this will signal to the unit that the temp is lower tthan 5 degrees  and it will pass voltage to the unit. and hopefully fire.usually people leave the fusr in   as when removed for the summer   sometimes the unit will seize up   usually though a thump with the hammer gets it going.  you can sometimes hear the fuel pump for the unit ticking   its under the central ara of the middle row of seats    a very common fault is the fuel pump wires corrode as they are exposed     re connection sometimes starts the heater. the glowplugs are £30 + at dealers     we sell them in our shop much cheaper.   they are much more expensive than normal glowplugs as they are classed as industrial use    only beru  the german firm make them and we have to import them for the best price.   hope this answers a few questions   most info is correct  but o doubt someone out there will know more than we do. this is a rough guide   not a expert breakdown. it can save you a lot of cash to change the glowplug  as a dealer will charge a fortune and may say a new unit is needed at £300 +