Car Seats

I'm frustrated. I was able to get both Marathons situated in the back seat of my 03 Acura TL type S and still have room for my mom to sit back there! So that's a yeah!

However I want to anchor the RF seat and can't figure out where to anchor it. The Britax manual said to check my car manual, which I did and it didNOT address tethering/anchoring RF. So Britax recommended I call my car seat manufacturer. Well I spent 45 minutes on the phone with them while they read the manual I already had to me then informed me that they can only tell me what's in the manual. Which frustrates me because if the manual was helpful at all I wouldn't have called.

Right now I've got it anchored to the rail that the front passenger seat moves forward and backwards on. The problem is I have no idea if this is safe and it means that the front seat is practically in the dash board. As I've said before there are 3 adults that need to fit in the car and so it's really uncomfortable for whoever sit there. Obviously we will just accept it if that's what's the safest but I just wanted to see what other options we have. I've attached pictures of the anchoring!

TIA!!