So this is something we know over in the uk so i thought i'd share with you guys as a few people asked me bout it.

What you will need
A. 1 blown heater resistor card
B. sandpaper
C. Some form of cleaning solvent (i used carb/MAF cleaner)
D. Soldering iron and solder wire

Method
1. Remove the heater card from the car.
2. When you remove it you will see the place it has burnt, there is a corroded rectangle of metal in the center of the card, sand this with sandpaper until you can see the two square contacts with a gap between, you are actually able to remove all of the old "resistor" by sanding if you are a sucker for detail.
3. Once you have unveiled the two contacts clean the card and prepare for soldering.
4. Solder across the gap between the two contacts, ensuring the solder grips well, if it doesnt stick to the card, sand it back a little more as solder wont stick to the burnt "resistor"
5. Plug into the wiring loom and test the heater, if it works, put it all back together and relax with four heater settings. if it doesnt, take it off, sand it back and start again.

NOTE : if the heater card itself has rusted you may not be able to repair it.

I'll update this tonight with pictures, im at work just now suffering from friday afternoon syndrome.

Hope this is helpful

Hey, got any pics of the process?
I salvaged 3 of these cards from the junkyard to give it a shot...lol.