I've done some scanning of my Haynes manual because I thought this might help a few people.
You don't have to access the TID from the wiring loom as you can get to it from the ISO connector behind the radio.
[gorgeous] I too have a nano and power it from the usb connector which plugs into a USB cigarette charger adapter. When i've made a final product, i'll buy a cheap usb charger and wire it into the loom behind the console, and attach a (very short) USB to mini-USB adapter.
I didn't mention in the PDF that the arrow J> is the Red/White connector ('gorgeous' put red/grey) that tells the display to display the date or the text string. This wire is just the antenna-power wire given out by the radio that is meant for electric aerials, which tells them to rise or drop when the radio is switched on/off.
My radio for example, turns this off when I listen to CD or AUX, leaving the display to return back to the date, and tells the TID to expect a string when it's powered on (because it would be receiving an RDS station name).
###Potential BUG with the arduino###
My nano is a bit of a pain if the power isn't steady. If it loses power or has a power ripple, you need to turn the arduino off for a couple of seconds before turning back on. There appear to be some capacitors holding charge that don't quite let the arduino power down and boot up properly, unless it loses power for a good 2 seconds. This might be why the blink program isn't even working sometimes, the bootloader doesn't load properly, causing all sorts of blkinking lights, but not the correct program to run.
Dan