I'm going to be doing some recording of conversations in a car - the majority of dialogue will be between the driver and front-seat passenger in a car; the camera will be behind them in the rear seat.

The person behind the camera will be the interviewer, and it would therefore probably be easy to do ADR for them in post. Nevertheless it would be nice if I could avoid this.

I'm recording to a Sony PDX10 - I currently own the following audio equipment:

Two Audio Technica AT803B Lavs
One Audio Technica AT897 w/ K-TEK K-SSM Shockmount (Camera mounted)

Now I'm thinking the best way to do this would be get a mixer of some kind, put a Lav on both the driver and passenger, and use the AT897 for ambient noise/interviewer track.

This, of course, would require a mixer though;
Budget for mixer would be under $400.

I have a a 12v to 120v adapter in my car (400watt version). In the past when I tried hooking a home minidv Camcorder (Sony PC100) into it to recharge while recording noise was introduced into the camera's audio, so I'm hestitant of powering a mixer in this way, unless I can ensure there won't be a similar problem with the mixer/ figure out a way to get rid of this noise.

Thanks,
Ari

The problem with using a mixer with 3 closely spaced mics in a loud ambient environment is that you really must use a person to actively mix the mics to get the best acoustic results. There will be too much cross-talk between the mics, as well as greater ambient pickup and the problems with sending 3 mics to 2 recording channels that makes post-production difficult if you don't actively mix. On the other side of the coin, active live mixing is prone to human error.
You may want to consider recording the interviewer to a separate device, like an MD or an iRiver. Have the two lavs go directly to the camera and split to left and right. If you place the lavs on the people, be cautious of seatbelt rubs and other clothing noises. If the car and road surface are quiet, you can place the lavs on each sun visor to avoid this.
You're going to get plenty of ambient sound with omni lavs. You should record plenty of plain ambient sound separately, including passing vehicles etc., to use in smoothing out edit points. You'll also need this ambient track to use with the interviewer if you decide to ADR him later.

Thanks Jay, I like the sound of your solution - it doesn't require for me to buy a mixer :)