Easy enough to fit , if you can find the aux wiring you could use that simply run the wires down and solder on.
You could put in a completly new switch but always use a relay. If you are unsure buy a pre made kit from Halfords...about 11 quid. They have instructions,the wires are colour coded so its a doddle. If you wire it yourself use 27 amp wire...no point scimping as hot wires = electrical fire...not good