Evenflo Symphony 65 Convertible Car Seat - Liberty at ToysRus
20% off coupons float around for this so don't pay more than $160
In rear facing position, you will need to use the strap on the back and anchor it to the front seat frame to help keep it level. All of the seats I looked at did not recline enough in rear facing position so the baby is too vertical. This was the most reclined seat/base available and is close to working, but you need to anchor that front passenger seat frame down low to get it properly leveled (as you cinch the anchors, it pulls the front of the seat down and the back of the seat up). Other thing, the base is not nearly as easily removed as an infant carrier. You can't pull the seat off of a base in one car and slap it on the base in the other car like you can with an infant carrier.
For an infant carrier (rear facing only): Evenflow Embrace LX infant car seat. This one fits really nice and is small/easy enough to get in and out and the base is fairly cheap so you can buy a second one for the wife- very convenient. Only downside is my baby is "healthy" and now exceeds the 20 lb weight limit.
In both cases, you use the supplied belt that secures to the seat anchors, in addition to the shoulder belt with the locking clip. Put your full weight on the base/seat as you cinch the belts. Grippy seat protectors can prevent excess sliding as well as save your leather. Like stated above, you should only have about an inch of wiggle each way.