A poorly built automobile. A piece of junk. The reason behind the name "Friday" is that workers typically had a long week, are tired, and are looking forward to the weekend... therefore, they are a lot more careless with the construction of the vehicle, lending to it's numerous problems.
This typically applies to a make/model of vehicle that is usually of pretty good quality, and the "Friday car" is more the exception than the rule.
Really bad cars can be referred to as "Friday afternoon at 4pm before a three day weekend car".
Yeah... must be a Friday car.