The whole premise of this thread is a mis-used word.
A car that has been "customized" does not make a "custom" car.
People like to talk their cars up in order to sell them, compensate for their own shortcomings, or generally just to feel like a big shot. What they are doing is "modifying" something that already exists to their own tastes.
If people don't know what these words really mean, they toss them around because they sound cool to them, or heard someone else mistakenly using the word. There are examples of this all throughout society. Just let the slow-minded use whatever word they want.
The internet has only helped spread this English-destroying disease further. Look at all the unintuitive acronyms/abbreviations that people use in real life like "O-M-G" instead of "Oh My God". To type it is faster, yes, but to SAY it in real life, is no faster, and only demonstrates how little that person actually comprehends about why they either type or speak in a certain manner. These are the idiots that our soft society has allowed to exist and reproduce.
To call any mass-produced car "custom" is wrong. It isn't custom. You didn't have the car designed to your specifications. Bottom line. You may have "customized" it (which is a partially custom vehicle, tantamount to modified, the two words mean the same thing). It's like taking a size medium t-shirt from Walmart, scribbling on it with a felt pen, and calling it a custom shirt. It isn't. It's not sized for you, it wasn't designed for you, it was designed for a huge amount of people that will want that shirt, and to whom it will somewhat fit.
My whole point: The word custom has been bastardized to mean something it doesn't mean. Go ahead and use the word incorrectly if you like, you only make yourself look silly to people who actually speak English. Good luck with that.