It has been covered a lot of times and can easily be found on the search
the heater resistor pack is behind the glove box. 2 8mm bolts under the glove box and lift it out.
theres a connector plugged into the top of a sort of lid. Remove the lid by unscrewing the two screws (the screws were 6mm bolts in my old streetwise but posi screws in my ZS).
the resistors will be broken. if you do a search you can find the correct values for the resistors, tootle off to maplins and pick some modern resistors up for pence. Solder them on and problem solved. this is how i fixed the streetwise.
Alternatively if you dont feel confident soldering your own, searh for a heater resistor pack on on ebay and replace the whole part (used this on my ZS and my friends cars just to cut out the hassle of soldering)