it looks pretty neat for a backyard job mate, well done.
but...theres a few things to consider from now on.
In Vic for instance, the VACC advises that this equipment, although reasonably low tech, has to be installed by a qualified electrical/electronic technician of some sort. that generally means an apprenticeship/traineeship at a workshop or tertiary course related to electronics. if you don't have the paperwork to back up your install their insurance may be void and if they stack their car with no insurance who they gonna blame....
secondly, if you're gonna be working electrically on cars, trucks, boats whatever...get some sort of business insurance. that'll save you if you make a mistake and the car gets damaged. for instance, if you wreck an amp cos it's hooked up wrong that could be $800 you have to cough up, the customer's not gonna pay for it remember. and get an abn...do it right first time and cover your ass
having said that i'm not assuming you haven't got all of the above, but you did mention its just a hobby etc between you and a mate
cheers and good luck