I bought this car harness which is crash tested:

http://www.amazon.com/gp/product/B0064ACNLO/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

And then used webbing that I used in search and rescue for helicopter liftouts (you can buy it at REI and other sports stores) to make a two point retention harness. Just tie the webbing to the child safety points in the rear seat and then tie them to a climbing carabiner. All you need to do when the dog gets in is clip the carabiner to the harness and you're good to go.

This is a very inexpensive retention system that leaves the dog a fair amount of motion but in the event of a car wreck will prevent them from moving too far forwards or up. They still have some motion side-to-side but that's going to be native to any retention system that leaves the dog room to move around or lay down a bit. If you don't want to have that then you can use the 3rd child safety point and another length of webbing; though, to be honest, my dog can only move about 6 inches in any direction which is just enough to let her be comfortable while still protecting her in case of a car wreck.

If my description doesn't help let me know and I can get a picture of it.