Overheating

[1falconer1]

I have a 1999 AU with 177k on the clock. Noticed recently that she was starting to overheat when towing a 1400kg caravan. After a few attempts with the local garage fixits came to naught, I found that the fans weren't coming on. OK, I thought maybe a fuse or relay. Local auto elec couldn't trace it but thought that the temp. sensor on the head might be on the blink - but he didn't know where it was! A good buddy rigged up a temporary fix where the fan is turned on when the ignition is turned on - a cool engine! However, I then realised that while the temp. gauge on the dash doesn't show any drop, the engine seems a lot cooler under the bonnet now than it ever has! Maybe the engine has been running hot for a long while and hasn't showed on the dash temp. gauge?
Q - where under the bonnet is the sensor for the switch to turn the fan on?
Q - is it possible that the temp. gauge on the dash isn't part of the setup to the switch that operates the fan and is therefore not a good indicator of temperature?
Q - the boiling is only a recent ocurrence, but the under-bonnet high temperature has been evident for a long time. Could there be a radiator blockage that effectively blocks off say 1/2 the radiator which leaves the temperature OK for general use with the fan operating, but towing a caravan becomes too much? (with or without the fan)

I'm 2 hours from a Ford dealer who charges $100 an hour. I'd like to be able to point a local guy at $45 an hour in the right direction if I can.

Any ideas anyone?