Thanks for all the advise guys. Trust me, the seat inspector and I were double teaming the seat install and just could not get it snug enough. I have 2 Britax Boulevards, one for the 3 year old and the other for my 9 months old. I think the problem may be as some as suggested that these seats don't fit the 80 rear seats well. I may swap seats with my neighbor and see if it's something that can be fixed with a new seat.
The Boulevard should work just fine. I don't recall the models we have off hand, but we have 3 different ones and all are similar to the Boulevard (I think two are a step up and one is a step down).
First off, try doing it in the middle first. The middle belt is
far easier to do than the side shoulder belts.
Second, I have never been able to latch more than one side on the plastic latches that go over the belt (to lock it in place and keep it from moving). You're supposed to do both, but the plastic latch on the side that clips in is too close and will interfere, so don't even bother.
Third, I've had the best luck by pulling the belt all the way out, then letting it retract. The buckle should look like it's 3-5 inches too short to reach the clip. Now without letting the belt retract (again, easier with the middle belt) throw all your weight onto the side
away from the buckle and clip. Torque down on the seat belt, and (if you have enough hands or a helper) latch the plastic retaining clip that's on the car seat. This basically forces that side of the car seat deep into the seat, and by latching the clip or torquing down on the seatbelt itself, you're preventing it from popping back off. At this point the buckle will be at least 5-6 inches away from the clip (even further than when you started).
Now put all your weight on the buckle side. You should be able to use leverage to lower the seat until you can clip the buckle in. The more difficult it is to get the buckle clipped, the tighter the seat will be and the less movement you have. I've done it a bunch of times and it still usually takes me a few tries. If you can buckle it easily, then it's far too lose and you need to start over, with less slack in the seat belt.
Forth, I assume the seats are rear facing? Are you using the top belt?
I've snapped the top belt onto the foot of both the passenger and driver side. You'll likely need to remove the plastic covers over the bolts. On the passenger side, on the inside foot there is a square hole cut out that you can use. I don't recall if there is one on the drivers side or not, but the foot has a lip on it that you can catch the buckle on. Once you tighten down on the strap the buckle won't come loose, unless somehow the strap is loosened.
On the newest one we got, it came with a loop for the feet. Clever little design, just basically a loop of strap. Wrap it around the foot, loop it through itself, and then clip the buckle into the loop. Even if the strap becomes loosened it won't come out, so it's a very good way of doing it. You could easily replicate this with a strap or some rope, but as I mentioned before you can also clip directly to the feet without any issue.
Once you tighten that third point down, the seat should be in tight enough that trying to move the seat will move the whole vehicle.
One final thing, some people find that their seat (specifically Britax, but others as well) fit better if they put something under the front of the seat. I've heard of a 2x4 or towel being used, but what seems to be common is one of those pool float noodles. This lifts up the front of the car seat (the part that goes against the back of the passenger seat) and changes the angle. This may (or may not) help you fit it better.
IIRC the Marathon's also have adjustable bases. So make sure those are adjusted properly, if they're not tilted all the way back they don't fit right.
second, I am trying to remember if you can recline the second row a bit or not.
Second row does not recline without modification.
