Get a 240-12V converter. Not very exy, Coleman make one and I'm sure you could find one in most camping stores. (look something like this
Then just wire up a neg and positive from the battery, to the neg, positive, and accesories wire on the CD, with a plug. (wire the Acc wire to the positive so when you turn it off, it doesn't loose its memory).
Don't stuff around making your own way of converting 240 to 12.
Because the convertor will have a Cig plug attachement, you'll need to work out which is +/-, and make up some sort of plug so you can either plug the stereo to the car battery, or to the convertor.
Hope this makes sense cause I typed in a hurry, but it shouldn't be all that difficult.