not sure if this counts as a modification per say, but its something i did..When i purchased my 2006 mazda 3 the rear mirror was broken(would not stay were you adjusted it)..i have a Saturn SL2 that i had kept the rear mirror for some reason so i measured the Saturn mirror and found out it was 1 inch longer and 1/2 an inch taller than stock Mazda 3 rear mirror..removed Mazda mirror and the Saturn mirror bolted right in its place..now i have a larger rearview mirror and better view to the rear of the car without blind spots..