This isn’t answering the question (I always call them Lexi when I have occasion to discuss them, which isn’t often) but I do know the reason why they don’t pluralize it on the commercials: you can’t pluralize a trademark. I learned this when I worked at Xerox–a trademark must be used exactly as is, and can’t be altered in form. That’s why you see commercials like the one that annoys me: “Got your Pop Tarts™?” “Yeah, I got it.” (can’t say “Pop Tart” because the trademark is “Pop Tarts”). So they can say “Lexus vehicles” or “Lexus brand automobiles” but not “Lexuses” or “Lexi” or whatever the hell it is.
That’s interesting, winterhawk11. I’ll start listening to other car commercials to see if they do avoid the plural.
Matters of great import. eh? 
Peace,
mangeorge