Ok, the problem I see here is that if you had an opportunity to inspect the vehicle and did so and still took delivery of it despite the fact that it didn't have the promised system listed on the invoice, then they can say that you accepted it with the options it had.
With that said, if they told you that it did have the requested nav system, it just needed downloads, then I think you could file a lawsuit against them for breach of contract based on fraud and misrepresentation because they misled you into believing that it had the correct system and then lied when you asked about it and told you that it needed additional software to function which turned out to be incorrect.
When faced with the possibility of being dragged into court, having to hire an attorney to defend, and then losing (since everyone hates car dealers anyway), my bet is that they will agree to settle with you and either put in the requested nav system or switch you to a new vehicle..
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