All the GPS does is go to earth for each wire, when the gear is selected.

So, just put it in first, find the earthed wire and note it.......repeat as necessary.

Then, find a bunch of LED's, but make sure you know their rating. Then google "LED resistor calculator" and punch in the numbers and it'll tell you which resistor to use. Easy peasy.

I'm no electronics expert, but the internet has all the info you need to pretend to be one.