2002 Focus Cooling System

[ooze]

My brother in law's 2002 Ford Focus is overheating at idle. I did some poking around with him this afternoon, and read a little on the forums here.

What I observed:
- The rad fans came on shortly after a cold start of the car. The cars heaters were however running as he'd been using them to help with the overheating. Once we turned the heaters off, the fans never came on again.
- I found the low speed fan resistor (I didnt actually know what it was when I found it), and it didnt appear "melted" as I read in some threads commonly happens.
- When the car was running cool, the 50A fan fuse was live on both sides, but the 30A fuse showed no voltage on either side. No fans were running.
- When the car was over-heating both the 50A and 30A fuses were live on either side.
- Coolant level was fine
- Thermostat was replaced recently
- When the car was overheating both hoses into the rad (in and out) were hot.

My best guess is that whatever is controlling the fan is failing, though I have no idea what this is. There didnt seem to be a fan controller switch, and the motors spinning briefly while the engine was cold (with the heater on) confuses me more.

Could it be the low speed fan resistor? This seems to be a common suggestion, though I'm not sure what this resistor does, and both fans are effected.

Suggestions/comments/etc would be very much appreciated.