It would sound awesome and it isn't a bad idea. As long as the tuning of the TL doesn't go below the longest mode frequency of the car (The longest dimension). I have played extensively with TL's in the automotive environment and the only problem with the TL is finding the space for such an enclosure.
Absolutely. All of the work that it takes to figure how the TLs will fit is definitely paid off with the nicer sound of the TL. You'll get even more loading from the TL because of the environment inside the cabin. Since you'll be using this for mid-bass drivers, you'll want to tune the TL near the xo point for the woofer (if you have one).
The thought that I always have about transmission lines is that they are periodic transmission lines in which people go to a lot of trouble to damp out all of the resonances except the one that occurs at around one quarter wavelencgth of the line.
There is already a device which has an inherant property of having only one resonance and that is called a reflex enclosure. Altogether a lot simpler to design and easier to make, and completely free of the series of resonances that is inherant property of a transmission line.
The difficulty of a reflex of course is that it lacks the mystique of a tl.
Reflex has many more than one resonance and higher excursion than a TL would have below tuning frequency. TL's have better group delay and better mid-bass response, this is why many like TL's and frown upon simpler designs. Reflex has resonances of the actual vent and with a mid-bass driver one would have to dampen this resonances from escaping into the listening area unless they like a cave-type sound.
A superior bass system (a way to control the back wave, ML-TL, reflex, etc, etc.) will always have a better mid-bass response (phase, frequency, CSD, etc, etc.) and this is one reason why TLs have the fanfare that they do.
IMO, TLs and Open Baffle Loudspeakers are superior in sound and if you can't do either then look at a sealed or vented enclosure.