I made it just in case of an accident (I recently had one, and the offending party lied through their teeth about what happened).
I used the MLG-RD21 ($30), which comes with a suction cup for your windshield, and cig.lighter -> Mini-usb adapter for charging the lithium-ion battery. It's not that great of video, but works for my purposes. I didn't like how clumsy the suction cup looked, especially with the wire dangling down to the cig. lighter, so I installed the camera to the base of my rear view mirror (that way if the mirror moves, the camera doesn't), and hardwired it to the constant 12v power in the roof interior lighting.
It records 5 min. videos, then loops when the mem.card is full and records over the first vid. I don't like the angle of the lens, so I've also ordered a 160* wide-angle for $12 each (it'll have more of a human eye angle, and see more of the traffic beside me). It's also audio-detecting, so in my garage it's not recording, but as soon as it hears anything, it'll start.
So in total the cost was $45 with shipping. Here's some pics/vid. I'll post another one when I get the wide-angle installed.
Camera MLG-RD21 (AA batt. just for size comparison)
In my Audi
In my Subie
And a ride in the Subie (Camera is mounted upsidedown, so it's more hidden. I have to rotate the vid 180*, hence the date at the top is upside down) I'm not sure what the deal is with the sound crackling like that, the other one in the Audi doesn't. Must be a defective mic.