I know this will go against the grain here, IMHO, dont get one.
If you are getting this to keep the kids quite/happy while driving, dont do it. When in the car, that is the very best time to TALK, to your kids, you have a captive audiance lol.
I have two boys, granted they are older and driving now, but I refused to get the DVD monitors when my boys were young, we "talked" to them, they never missed the tv or there shows, we talked about everything, it was awesome.
Heck sometimes when going to the store, we would go to one 30 minutes out of the way, just so we could chat with them and have thier undivided attention, they seemed to love it, cause they would ask us anything and everything, and they, had our undivided attention..
(flame suite on)