Hehe, i've been building industrial PCs for about 1,5 years before i started stdying. We were using ADVANTECH Boards, mostly.
I had very little problems with those boards (5862, 5864, POS460, POS560 etc)
They have a very small (size of 1,44" floppy) 486 Board with 133 Mhz. This, combined with a FlashDisk of about 16 or 32 MB (Use Drivespace or other compression!!!) for an OS and a CD drive should make a perfect MP3 player for your car.
What i didn't find, was a good 12V to 5V converter to get the power from.
And YES, these Boards ARE expensive. About 2 times the A7V cost for this 486 or 586 board ... at least here in germany.
Most of these have integrated graphics (CT6555XX with LCD and CRT connector) and flashdisk, as well as watchdog timer (useless for normal people), RS485, RS232 and parallel ports. HDD and FDD controller are there, too.
Advantech Support in germany is good (Hi Mr. Wiensgol

)
But if you want to PLAY and see DVDs ... now ... get a real board. CT Onboard graphics suck for that. I don't know how you'll get the 220V (110V?) power for a devent power supply though, and I don't know what type of display you plan to use.
Hmmm ... any questions

?