Oh, only $130, I thought it would be a lot more.

The A/C amp internals are all discrete components and it can be repaired.  Mine was wrecked by an incompetent A/C mechanic trying to get a stupidly overgassed compressor to fire up by blindly pumping 12V into any connector that seemed A/C related.  The fault ended up being a transistor connected to pin 10 that switches the no. 1 and 2 fan relays.  A simple fix and 5 years later is still working.  PM me for more info if you're interested.
 
BTW, the diode you mentioned is there to short out back-EMF from the relays (a high reverse voltage created when they de-energise) that would otherwise damage the transistor in the A/C amp.  I think it's taped to the loom somewhere near the heater box.  You don't need to find it to test it - with the relays and A/C pressure switch unplugged you can connect a diode tester to the coil connections on one of the relay plugs.

robk - sorry the exact online saving price is $135.28, the msrp is $173.08
 
cogs - whoops 'shorted' was probably the wrong word to use more like fried! Behind the circuit board where those little black chips are, there's a small area there where a few of them are burnt out. I'll post up a pic when I can, dam battery went flat on the camera. 
 
Burnt out area is behind that circled resister.



 
 

Where exactly is the A/C amplifier? Maybe I should pull mine out and have a look regardless of what else I check. All I know is it's under the dash somewhere.

Not too sure myself but if i have to guess it just turns on/off diff parts of the air con system (inc rad fans) based on the signals it gets from the sensors/switches in the system. Its located on the passenger side, behind the heater bulk (sh!t a$$ location). Its attached to a bracket which is screwed onto the heater bulk, just unscrew the two screws, once you do that try to loosen it and disconnect the harness via feel because its quite tight down there. Once you get it out you can just unclip the top and slide the circuit board out.
 

Thanks for all your help mate!
Now I have plenty of things to check once I get a chance.

oh yeh just noticed that your car is a 1990 the locations should be the same but the part number will be different, since they changed the air con system after the 92 models.

i've had numerous cars with simialr faults. there are several versions of the a/c amplifier and i would strongly recommend you remove it and get the part number off it before ordering. the location changes slightly between models, but its always on a bracket up behind or beside the a/c box under the glovebox.

Just an update, got a replacement a/c amp from ebay, installed it, now the fans are quiet :) I guess its fixed

Nice one :-)