The valuation of a vehicle is a constantly changing one. Your best bet would be to create an amortization table based on your current loan terms and start date and cross reference that with valuations of older models. This will allow you to see where the two cross paths. It won't give you a %100 correct answer, but it will point you in the direction.
You can find a amortization table here: http://www.bankrate.com/calculators/auto/auto-loan-calculator.aspx
I like this one as you can see the effects of adding extra payments.
You can find valuations at http://autos.msn.com in a quick format by years that seems relatively close. But have found http://www.nada.com more accurate at least for reasons that I have needed valuations.
Good Luck!
