We have the same problem. Previously we had rear facing Britax B-Safe Infant Car Seat placed on the passanger side of the back seat and we didn't have any problems. Now our son is too big for that seat and we had to buy bigger car seat (Britax Marathon 70-G3 Convertible). I tried to put car seat on the same place than before and follow the instuctions. We still wanted to keep rear facing installation. I tried to use tethering spots on the trunk, but then we were not able to get our son in the seat, because the tethering starps were in the way. There is no "official" tethering spots below the rear seat, so I just fastened the strap on the horizontal support structure of the rear seat (below the seat). That structure didn't feel very solid, but I still think that it was the best way to tether the seat. What do you think?
BTW, I noticed that base of the old car seat streched the leather seat quite a bit and lower anchors also made the seat look pretty bad. I hope that leather will recover when car seat is removed. Are there any tricks how to make car seat look new again?