Ok I just spent 3 weeks playing this, Just got my last item and whaddayaknow - No achievement, FUCK this game, Avoid it like the Plague. It's unfinished with a pathetic amount of content for the money you pay, The poxy Dev's couldn't even be bothered to answer when I found the last achievements were broken and I can't be assed to let them know about this one. I really hope they go under. Codemasters are dead to me and I will never buy another one of they're games and I hope you won't either. Sorry if you came here looking to find an easy way to get this, Just a heads up, I played over 900 games coming 1st or 2nd 95% of the time to get all the items.
UPDATE-
MemoriesOfFinal has found out that if you change a few items on the vehicle you have completed then back out from the main menu then reconnect it may pop, You may have to do this several times.
UPDATE AGAIN-
I seem to have found an awesome glitch!!!!
Go to the garage and change an item on any car then back out from the main menu and reconnect, Look at your money- Yes you have 10 bucks more, Now repeat until you have enough to buy every item for a vehicle, FREE MONEY Y'ALL- Your welcome
UPDATE-
Turns out the money glitch only works if the achievement glitches out on you, So yeah 900 games or maybe a 2x weekend will be needed for this achievement after all.