How do I wire up a Bathroom light on a pull cord switch?

I need to replace a bathroom light on a pull cord and here's the problem.. The ceiling hole for the light has 3 Red, 3 Black and 3 Yellow/Green wires. Each time I have tried to wire up to the light instructions provided the fuse blows. Once wired and I put the fuse back in, the light is on. As soon as I pull the cord to switch off, the light goes out and the fuse (in the fuse box) blows taking all the downstairs lights with it. The bulbs don't blow and as soon as the fuse wire is replaced all is fine again.. I am told that there is a certain combination of wiring that will solve this and that this involves mixing some of the red/black wires? Can anyone help?