I think Margaryta is on the right track, but I think the idea of tuner is slightly different from personalized. To me, personalized just means adapted to meet the owner's taste, but tuner implies a whole car culture that embraces various key characteristics.

It can be for appearances only (big wheels, huge spoiler on the back, kits to change the appearance of the body, adding non-functional features for the look only such as a fake hood scoop or tip on the exhaust pipe). Or there are people who actually make modifications that change a car's function. They put in the real racing exhaust system, hey modify their engine and transmission control modules to change shifting characteristics and fuel mixture. They might have a spoiler that's adjusted correctly, not just stuck on the back.
The problem is, making these modifications takes a lot of money and talent to do them properly. Some car makers are starting to make cars that look and sometimes function like tuner cars straight off the assembly line which appeals to people who don't have the time or money or ability to create their own tuner. I think the translation should keep the word "tuner", because it implies more than just personalizing a car.
My guess for manufacturer would be Honda. I know Civics are really popular tuner cars. Possibly Nissan. I could be totally wrong, but I don't usually associate luxury brands with tuners.
I would say Volkswagen, since the new GTI is "pre-tuned by German engineers" :-) But we're not talking about European companies here.
By the way, a lot of the Japanese tuner cars are called "ricers" or "rice burners" or "rice rockets", etc. It's a slightly derogatory term, often times used to make fun of people who try to make their car look tuned when it actually isn't (the people who do it all for show, not performance). If this source is speaking negatively about these types of cars, ricer could work too. But tuner is more neutral, not many negative connotations come to mind.