how about a try to find out?
buy a car twice and on the first you install all starting with the ecu
on the second you install all ending with the ecu...
it doesnt matter. in real life youd do it last but i really doubt it matters in gt5
If I remember correctly in GT4 it did matter what order you installed the parts in, for the most power. I'm still trying to figure out if that matters in GT5, and if there is a break in period for the cars.