The pronoun "anata" is the supposed neutral way to refer to someone whose name you're not aware of, and it's OK to use it to a stranger if you can't think of any other way to phrase the thing you want to ask.
The main reason why it's so frequently warned against is that the first instinct of speakers of English (and other Western languages) is to use the word "you" in almost every sentence, when it isn't actually necessary in Japanese, and therefore will sound awkward. Native Japanese speakers won't generally resort to "anata" unless there's really no other option, and in the vast majority of situations, there will be a more natural way of phrasing whatever it is you want to say.
For most everyday sentences, it will be possible to avoid using any pronouns at all, simply using honorific language to make it clear when you're referring to the listener (as in the basic "onamae wa?" where there's no need to say the "you" of "what's your name?" because the honorific "o" already indicates whose name you're talking about.) If you can avoid addressing the person by name at all, so much the better.
If mentioning the name is unavoidable, there are a number of alternative strategies that will usually be employed before falling back on "anata". For instance, if you're employed in any kind of job where you have to talk to strangers on a regular basis, it will probably be appropriate to consider them a customer and address them as "okyaku-sama", so "anata" will never be used.
If you're talking to random strangers on the street, depending on your relative ages, it may be natural to address them by terms like "ojii-san", "obaa-san", "oji-san", "oba-san", "onii-san", "onee-san", "ojou-san", and so on and so forth. (Most of these will come across as somewhat familiar, and so might not be a great choice in all situations, but they can definitely be more natural than "anata" depending on how friendly-versus-polite you want to sound.) For children who are much younger than you, you would generally go for "kimi" rather than "anata" anyway.
Finally, if you're addressing someone whose name you don't know yet because you've just met them, there's a tendency to avoid using "anata" by instead making a slight pointed hesitation and trailing off when you reach the point in the sentence where you would ordinarily say their name. If you do it right, they'll pick up on the cue and introduce themselves, allowing you to carry on using their actual name.
Now, for the actual sentence you brought up in the question (asking a service staff member which car is theirs), I'm not actually sure off the top of my head what strategy would be employed. This seems like a somewhat unusual situation (it would be much more common for the service staff to be asking you, in which case the easy "okyaku-sama" choice is available). Depending on the type of service worker, it might be appropriate to to call them something like "ten'in-san" after their position, or even "onii-san" etc. if the setting isn't too formal. If there's no obvious option that seems natural, then it might finally be appropriate to resort to "anata". (Though there may well be yet another alternative strategy I'm failing to cover!)