Easy to do. Cheap to do.
Will allow you to control your cars temp to a degree....
You will be able to turn your radiator cooling fan on and off as you please. Whilst keeping the safeguard of it auto turning on a the preset temp.

Tools needed

A decent crimper
Cable cutters/side cutters
Selection of Yellow and blue crimps (8mm ring, Push ons and In lines)
4mm and 2mm cable, various colours,
30 or 40amp 4pin relay
8mm or 10mm Spanner (depending on battery)

How to...

Firstly, locate the wire to join into to override the fan.
Working in the egnine bay. Look down the bottom of the radiator and find the temp sensor. It will have 3 4mm wires coming from it. Red, black and Brown/white (This is what it is on mk 3 astras maybe diff on others)

You need to cut the brown/white wire (if there isnt a brown/white wire there, trace which 1 of the 3 goes to the fan. this is the 1 you need).

Join a 4mm wire to it, using a in line crimp, you should just get 2 4mm wires into a yellow inline.

Run that wire into the cabin.

Wire up the relay as shown. You will need to run a live feed to the switch. Ideal place is the back of cigerette lighter.

So to recap...
Pin 30 To ground (earth bolt or batt)
Pin 85 to ground (earth bolt or batt)
Pin 86 to Switch
Pin 87 to Rad. (brown/white usually)

Switch pin 1 to Pin 87
Switch pin 2 to Live. (Eg cig lighter)

Pic to follow........