This is a real easy guide, you don't even really need to be handy with a soldering iron.
You will need.
LED Holder (N91AX from Maplin)
5mm 12v self flashing LED, I chose Blue. (N65AJ Maplin.)
Wire.
Heatshrink
Scotch Locks.
Method.
Disconnect the battery.
Pick where on your dashboard you...