Well, if you know you have a Q of .7, you don't need the anechoic response, but personally I would still test it.
What (most) people don't realize is that you're not really looking for resonance, but rather cabin gain. Well, for SPL purposes anyway. You want to find the frequency at which you seee the most gain over your baseline (anechoic) with your in-vehicle testing.
For example... I did this with a 1cf and single 10 just like you, but I didn't know the Q. I tested in vehicle first and found that 37hz was the loudest freq at my given volume. BUT, after testing outside the vehicle, I found that my largest cabin gain was at 43hz.
It's a handy tool and useful knowledge, but so many other variables come into play to squeeze thenths out that a lot of that may go by the wayside when you get into full tilt testing. For instance, the size of the enclosure may skew the resonant freq by a couple hz or more. And that's just one of about 1,000 variables. Lol.