Follow instructions for the fitting of this switch..
Earth = wire and terminal into chassis
Positive = illumination wire, you'll need a ciruit tester to find the wire that has power when you turn on the headlights. You can make your own basic tester using a bulb holder and bulb with the 2 wires connected to it, earth one of these wires and use the other to find the illumination wire. There's quite a few of them so it's just a matter of going through the plugs one at a time and turning on the headlights to see if you get power. To narrow the search, there'll always be a illumination wire going to the stereo so this is one place you can look and there's more just under the steering column.
Power out = the driver's side reverse light. Run the wire from the switch all the way back to the reverse light. It's a bit fiddly but you'll find a small gap under the vertical carpet behind the drivers seat. There's a thick loom taking this route to the boot already so it's not difficult to find. The hard part is to push the wire though from the cabin side and then pull from the boot side. Be careful that you don't leave the wire loose as it might catch on the removeable metal back panel. Best thing to do is to cable tie the wire to the existing loom.
Once you get the wire to the rear light, pop off the bulb holder panel at the back of the rear cluster. You'll find that there's two wires going into the reverse light, black is earth so leave this alone. The other is the one you need to cut and join the wire from the switch to. I always make sure that I cut the wire leaving enough to use a bullet connector as this means the the rear cluster can be removed without the need to cut and rejoin the wire again.
I've yet to do this on my car but I've done it many times on others. If you're not in too much of a hurry, give me a couple of weeks and I'll do another write up with pics.
thats what ive already got and im trying to replace it with a oem fog switch, i was fiddling to replace it with one and i blew a fuse ( 10A light fuse) in the process of changing it,I replaced the fuse and now cant get the fog to to work. no other fuses and the bulb have not blown
from looking at the ebay link the fog switch should have +,- marking inprinted on