You need a new transistor in the fan speed controller. The TO-3 case sits in the air duct to keep it cool. Good idea except that moisture, leaves and crud accumulates in the duct and rusts out the case. The transistor in question is a Motorolla 2N5686 available at around $12 from World Wide Electonics at http://members.iinet.net.au/~worcom/
No other electronic component suppliers in Australia appears to stock it.
When you replace it buy and fit a T0-3 cover kit from Disk Smith, Altronics or Jaycar and also spray the outside of the case with electronic spray resin. Easy to replace if you can solder and desolder. Unscrew case from firewall (shiny cast alloy near glove box), take case apart, desolder transistor and solder in new one. Clean out the duct while fan speed controller is out.