Hi and welcome,
You're lucky i was doing a bit of research into this myself this morning
If this is the one you are referring too...
http://www.ebay.co.uk/itm/Fiat-Gran...55702407-/111484787624?_trksid=p2054897.l4275
Then this is what i'm going for. When the heater only works on set 4 then it means the actual resitor (the bit above the green bit) has blown. If the image on the advert is what you get then all you need to do is unplug the resistor pack (the black plastic bit is the plug) and then plug in the new one.
So in short, you should only need the £8 part if your heater only works on 4. If the heater doesnt work at all then it could be down to a wiring issue, burnt wires behind the dash board are a common GP fault
Hope this helps! Would be intresting to know how you get on