My car system may be a little primative, but it works.
I bought a used IBM Thinkpad and two 200g HDs. The Thinkpad is hooked up to car stereo's aux input from the HP/speaker output.
For power, the laptop runs off a Radio Shack power supply. The HDs are in the trunk with a power inverter.
I put in a couple of switches in the dash to turn the 'puter and HD's on and off and an ignition bypass switch. (These switches aren't neccessary, but they give me more flexibility).
After having started up windows (with the hard drives powerd up) and the laptop set for "standby" when the lid is closed, I close the lid and turn off the ignition. The laptop will be in standby running off it's internal battery until I start the car (powering up the HDs first) then open the lid and presto, the music picks up where it left off.
The sequence is critical, power up HDs first, then turn on laptop, the reverse to shut down, otherwise you have to restart the computer and with win2k and this old IBM it takes about 4 minutes to boot up! I could be at my destination by then.
I have left the laptop in standby for 4 days so far, of course the internal battery recharges when the car is running.
I don't have quite as big a library as Mastiff, I have around 30k songs in the car.
Of course the car stereo is now worth about 3 times what the car is worth