Convertibles are not required to have top tethers, but LATCH is! LATCH=Lower Anchors and TETHERS for CHildren. I'd double and triple check your manual. My husband has a 2004 Audi A4 wagon and his top tether is RIGHT behind the headrest. And I mean RIGHT behind it. As in you have five or six extra feet of tail from the Marathon tether. It's RIGHT behind the headrest.
Have you tried installing it with a seatbelt? The Marathon is a pretty forgiving seat. I can get our Wizard (same size and shell as the Marathon) in with a seatbelt. And if you do a seatbelt installation the top tether is not required.
Some general tips. When you are installing the carseat, thread the seatbelt through the belt path and buckle it. Then tighten the seatbelt as much as possible. Put your body or at least some weight, a hand or knee pressed hard, into the seat. The part of the seatbelt you want to tighten, that's doing the carseat holding, is the lapbelt portion. So pull on that HARD and get the lapbelt taunt. Then route it through the forward facing lockoff and lock it. For an added measure of safety I then pull the shoulder belt all the way out and then release it, which switches it from an emergency locking retractor (which is like the front seats, it locks only if you suddenly pull on it) to an automatic locking retractor (which locks no matter what you do, even if you just exhale your seatbelt gets tighter). Then check at the belt path. If you get less than 1" of movement AT THE BELT PATH both side to side and forward and back you've got an acceptable installation.
Another option, if your child is less than 33 pounds, is to install the carseat rear facing. A child may prefer that so the wind isn't completely in their face anyway. Since older rear facing kids don't need a 45 degree recline you can put the seat pretty upright and you can brace it with the front seat (which is good, since I know A4s aren't known for their roomy rear seats). Then you can tether it down using the D ring that came with the seat. On my husband's A4 I found an acceptable tethering location by sliding the D ring under the track for the front seat to slide on, then I pulled it back toward the backseat so if the front seat was moved it wouldn't fray the D ring. Again, given the size of the A4s back seat, do this before you install the seat. If you do a rear facing installation you can do either LATCH or use the seatbelt. Whichever works best. And I'm positive the seat will fit in there as my daughter rode rear facing in her Wizard in my husband's A4 for quite a while.