When the seat is forward facing (ages from now since your little one is still little
) the tether strap comes from behind the seat and goes all the way to the back of the lift gate where it attaches to the tether anchor located there for the middle position... huge hassle so you'll probably want to move her/him to the outboard position at that time. With a Britax seat, you'll be able to rearface her/him to at least age two if not longer (many people go to age 3 or 4), so you've got a while before you need to decide on that.
For rearfacing (the stage you're at now), it's really quite simple to install using a seatbelt. LATCH is NOT safer than a seatbelt install. What's important is that you get less than 1 inch of side to side AND front to back movement AT the belt path with whichever method you use to install your carseat.
With the seatbelt and say you get a Marathon...
It is a lap/shoulder belt in the CX-7. Make sure the seat is in the recline mode. Pull the bottom of the cover and foam away from the rearfacing belt path. Open the lock-off furthest from the buckle. You will run the seatbelt under the LATCH bar through the lock-off opening and up through the RFing belt path. It goes under the cover and under the seat foam between the crotch buckle and where babies bum sits and then back down through the RFing belt path and under the LATCH bar. Buckle it up and then kneel in the seat while pulling all the slack out of the seatbelt. Position the shoulder belt in the lock-off with the lap and shoulder portions staggered of each other and close the lock-off. Check for movement by using one hand and tugging (gentle tugs, no hercules effort here.
) the seat AT the belt path. If it moves more than an inch, then you need to pull more slack out of the seatbelt and try again.
After you've got it in nice and tight, you want to take the d-ring that comes with your seat (when you buy new it's generally attached to the end of the tether strap or in one of the pouches where the LATCH connectors are stored. The d-ring is a short piece of seatbelt webbing that has a metal D shape at one end and a seatbelt webbing loop at the other. You take your d-ring and wrap it around an immoveable object... like the seat track leg. Thread the metal end through the seatbelt end and you've just created a tether anchor point. Attach the tether anchor to the anchor point you've just created and tighten down the tether strap. This will also adjust the rearfacing angle so you don't need to worry about getting the angle right when installing the seat since the rearfacing tether will do that for you... big bonus for carseats that can be tethered rearfacing!
Okay... so those instructions sound a little complicated, but once you have the seat and have read the manual and played a little bit... it really will be easy.
So, when rearfacing the tether is going to come down the back of the seat and attach to a point somewhere near the floor to one of the front row vehicle seats. Your child will not be able to reach the tether strap to play with it at all.
Hope that made a little sense and I didn't totally scare you! It's all about trying it out and you'll do great!