IP rights
While it is true that patent protection must be kept up, the same is not true of copyright. Copyright USED to require registration but hasn't for sometime. Renewals are required at certain timeframes (when the common law expiration of copyright would take effect otherwise.)
http://en.wikipedia.org/wiki/Copyright
http://www.copyright.gov/circs/circ22.html
However, I believe there may be a different situation in the case of the 'cars.'
Since copyright covers books, musical recordings, software, photographs, paintings and other visual art forms, plays and other performance arts, I'm not sure that copywrite is the right format for protection of a 'product' such as vehicle modifications (I may be wrong about that...see my statement at the end of this post.) Those may be under 'patent' protection. If so, as I said before, patent licenses must be kept up or they can be lost (read the part about maintenance fees.)
http://www.ipwatchdog.com/patent/patent-cost/
Now...I'm the first to say, I'm not an intellectual property lawyer. If you really want the scoop on what is legal and what is not legal, go to one of them. You'll pay some bucks for their expertise, but ultimately, you'll have the answers that fit the case you're questioning.
There is a certain amount of knowledge common to intellectual property (including the information that I learned in school which is what I've been basing my information on) and pretty much this is it in a nutshell: if someone else created it, there's likely protection available to them so be really careful before stomping on someone's intellectual property rights just because you want to do something with them (whether for fun or profit.) Legally, one can get up a tree rather quickly and if the person that owns the intellectual property is protected, ignorance of the actual law (e.g. going off of conjecture as to what's legal or not) will not protect you. Going into court with a defense of "I heard from some people on an internet forum that it was ok to do this or other people were able to do this and didn't have any problems" is just asking for a judgement against you. Sure, the intellectual property owner may never take you to court, but you have to ask yourself if you *really want* to take that risk. Up to you though. Just don't walk in blind.