Mrskylighter
Here is a little circuit which will work if its the 12v being switched in the plug. excuse my messy scribble.
Basically how it works is you have a normally closed relay supplying 12v to the a/c idle valve all the time whilst the ignition is on.

• When the A/C is switched off inside the car the ecu is sending 12v to the relay coil which stops the 12v being sent to the valve. (normal idle speed)
• When you switch the a/c on inside the car the ecu stops sending the 12v and the coil goes back to the normally closed position which sends 12v to the valve (which increases the idle speed)

Disregard where I have written blue a/c plug on my drawing. Its actually black :)
 
Maybe someone else would like to confirm that the below will work?
 

 
Use one of these below for the relay.

 
 

Awesome, thanks mate. I've just noticed something else... The two wires leading into the AC vsv plug are black and red, however current seems to be coming out of the black wire when the AC is off(I grabbed earth from another earth lead). Shouldn't it be coming out of the red wire?

No idea about what colour wires do what sorry. The BGB electrical section will tell you though. I wouldn't worry too much about the colour of the wire.

Mrskylighter
So just to confirm:

• With the AC switched off
  • There is continuity between the Neg pin on the plug and the car body?
  • There is 12v at the positive pin on the plug?
• With the AC switched on
  • There is continuity between the Neg pin on the plug and the car body?
  • There is no 12v at the positive pin on the plug?

Nope, other way around.
AC off: there is continuity at the neg, and there is 12v at the plug.
AC on: there is no continuity at the neg, and there is 12v at the plug.
In other words, there is always power at the plug, the ground gets switched.
Sorry, I should have explained that more clearly before.

Okay this circuit diagram is the one you want to reverse the neg/ground at the A/C idle up valve.
 
Once again happy for someone to critique my design to confirm it works or simplify it.
 

 
So with the a/c switched off in the car the neg/ground will not be sent to the a/c valve as the relay will be energised.
With the a/c on the relay will not be energised. As a result of the normally closed relay the neg/ground will be sent to the a/c valve which will cause it to operate.
 
I will delete the earlier diagram so as not to confuse anyone who finds the thread in the future.
 
 
 
 

i believe the reason it happens is because the a/c amp in the early models puts out a different signal than the a/c amp in the later cars. 
its the a/c amp that tells the ecu that he a/c is on via a/c pin on the ecu harness. 
 
if you build the cheap relay above you will be sorted. this idle issue is not due to a fault, just a difference in electronics.

Yep you can do the fix at the ecu harness instead. That's where I did it on mine.

You just need to decide where you are going to chop into the wiring.