"Si j'ai voulu cette voiture"
In English, when talking about conditional something that didn't happen in the past, we normally use past perfect, so "If I had wanted" instead of "If I have wanted."
Hm, here I would translate it : "If I wanted this car"
Meaning : "If I wanted this car, it was for the seats in leather."
Also, it's "passé composé" here, usually translated with simple past.