You Need to upgrade the vehicle Parts skill up to grade A in the facility menu, (this can also be used to gain the Tooled Up Achievment-Upgrade 3 facilities to grade A) When you have bought your upgrade, Start a Race with the car you want to upgrade by hitting upgrade in the service area. Now simply buy all the Grade A parts which will cost about 60,000, As this was with the Ford Fiesta. May be more or less