We are talking about cabin resonate tuning freq? I don't think that anyone will know exact freq. I was reading some info in TL forum. So findout something //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif If car cabin is small so freq is high, 55-60Hz, but in very small cars //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif And your car cabin is bigger so basically freq should be at about 45-53Hz. But not sure just guessing...
If you can find someone who have TL in your town you can find out that freq...