Good man Neil :) Yeh was hoping to use the same bulbs. Will Maplin sell the relay?
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Ok bloody Oven packed up tonight so only just manged to get it fixed
SPCO = Single Pole Change Over relay, its got 3 contacts called Normally Open (N/O), Normally Closed (N/C) and Common (Com). The Normally bit means what the state is with no power on the realy coil, N/C is connected to the Com, N/O is not, power on the coil the reverse N/C is not connected and N/O is :D
So if you wire the power from the brake switch to the Com and the feed to the bulbs to the N/C the lights will work as brake lights.
Take the switched live for the fogs to one side of the relay coil and connect (Delete "another wire") (Insert " a diode") from the same terminal to the N/C terminal. Connect the other side of the coil to a good ground, when you switch on the fogs the N/C contact will open breaking the circuit for the brake lights at the same time as feeding power to the bulbs and you got fogs on all the time :D
Maplins will do the relay, quote them "RS part no 351-639". Wired as above the coil will only draw 40mA so should not overload the fuse, the contacts are rated at 16A so that will give a max of 96 watts per bulb for brake lights, should be plenty.
You will also need a relay base to plug the relay into, quote them "RS part no 400-9130", this gives you the screw terminals to connect to. It also lets you change the relay easily if it packs up.
Total cost from RS £ 4.95+VAT, Maplins should be about the same or a bit less :D
With a bit of sticky velcro you should be able to find a place to hide it in the boot lid.
Sorry I didn't get time to do a diagram, if the above is not clear let me know and I will "nick" a bit of work time tomorrow to draw it up.