Hi Steve,
From memory the ballast resistor is located behind the air intake vent on the bulkhead.
The ballast resistor works by controling the total load imputted into the electricery to control the motor a various speeds, when you drive forward air gets forced into the intake and can speed the fan up which in turn speeds up the motor which is being supplied with electricty to turn at a slower speed than it currently is....the way a motor works is to pass the current through a coil to generate magnetisum which pushes against the field coil magent on the motor case.....this current then goes onto the next coil to generate another magnet etc and this pushes (or pulls depending on how the motor is wound) round. If the motor starts to go faster than the magnetisum pushing (or pulling) it then a process called back EMF (Electro-Motive Force) is generated and this gets fed back into the system which the electrical system wouldn't like so a ballast reisiter is fitted to control the motor speed but is also fitted with a diode pack to prevent any back EMF from shorting the system out.....
Hope that is as clear as mud.....it is behind the air intake grill on the bulkhead by the bonnet....
Rgds,
Saint.