I want to build my own cable with the ipad jack on one end and a guitar cable input jack on the other. I have an old ipod cable (USB on the other end). The connection points on my ipad cable are different from the ipod cable - but would the cable work on the ipod?

Here is what all the pins in the 30pin connector do.
Apple iPod, iPhone (2g, 3g), iPad Dock connector pinout and wiring @ pinouts.ru

Pin 5 and 6 are audio left/right input.

But don't electric guitars require Power? hence the name ELECTRIC Guitar? I think the amp normaly provides the power.

Greetings, not sure if you still need an answer, but they do make a cable exactly as you would like. iRiffPort & iRiggPort are the two... depending on what you want to do would determine which you would choose. Example... iRiffport will allow you to hook to an amp/speaker sound system to play thru the apps that can be bought thur AppStore. iRigg will only allow you to hear your playing as you play and/or play along with your tunes. There is no 'line out' on iRigg to allow for sound system hook up.