Splicing lighnting connector in place of 30pin to achieve video I currently have a car headunit that uses a combnation of USB (audio and control) and 3.5mm jack (video) that was designed for iphone 4/4s.

I now have an iphone 5. which i can't use the current cable with removing my ability to deisplay video. audio and controls still function fine.

My question is. I if i cut off the current 30pin connector off and spliced a lightning connector onto existing cable in its place. Will this cable function properly. Would it be as easy as, colour matching the two cables or is there a lot more to it. Could it be done with correct pin-outs?
No, it will not function properly. The Lightning connector is quite complicated and dynamically "rewires" the cables, so you can plug it in in either orientation and it still works. This makes it practically impossible to do a DIY rewiring.

You can get a Lightning to 30 pin adapter to use your iPhone in the car instead.