which relay should I use 4 or 5 pole and wht is the difference. Also do you earth one side of the toggle switch or the relay ?

signed confused :o:o

No real difference. You just get two terminals that are connected. Have a look on the circuit (should be on it).
They're cheap, so I usually stick with Bosch. Depending on where you get them, there are bases also available to plug them into for a neat instalation.
Or if your really keen (like me) there are 3 spare sockets in the fuse box. Just get some terminals to add in!!
:thumb

ok , but what about the earth. not electrically minded

is 6mm wire a bit big,what about 240v 2.5mm power wire ?

240V wire has thicker insulation to "hold in" the higher voltage. You don't need thicker insulation, but you do need enough copper to carry the high current.

Is 6mm the diameter or the cross-sectional area? Is it referring to the entire wire (copper and insulation) or is it referring to just the copper? These thing will make a big difference to what 6mm actually means!

I think you said in the other thread that the heater is 250W.
250W / 12.5V = 20A
So you need wire with at least enough copper to carry a 20A current (continual, not just peak).

Go to a shop like JayCar or Dick Smith and look at the wire they sell: they tell you what current each size is rated for.

Finally: make sure you put a heavy-duty in-line fuse in the wire. The fuse rating must NOT be higher than the wire rating: if a short-circuit happens, you want the fuse to blow before the wire catches fire.