Ok here's an overview of what was wrong with my ipad... This image is of a scrap board I had which I've removed the metal screening from. The red circle indicates the position of the short circuit capacitor (It's already been removed from the board in the pciture). Note this is the underside of the circuit board (i.e. that the side that's face down). Its easy to check with a meter, if its short circuit then this is you problem. This second picture shows the output from the LED drive circuit, the arrow indicates the point to test with a volt meter to confirm that the fault isn't with the voltage part of the circuit (The voltage should be around 18volts). You should be able to test this in place (you don't need to connect either the digitiser or LCD to check it), my tip is to use some pins soldered to your probes to make it easier to test. This is the topside of the circuit board, you should be able to access this whilst the circuit board is installed in the ipad.

Have just fixed an ipad with no backlight. After much work I tracked the fault to a short circuit capacitor on the logic board this held one of the gate inputs to a mosfet low and hence turned off the backlight driver circuit. I also have figured out most of the circuit for driving the LED backlight.