I have two smaller labs, so not sure that my info will be too helpful to your big Dane. But my boys wear seatbelt harnesses (not walking harnesses) purchased at Petsmart. Each is clipped to an 18 inch long, 1 inch wide leash. The leash is clipped to the floor tie-down by looping the end through double caribiners. I use the front floor tie-downs on my JKU, one for each dog. (My back seat is folded flat and stays that way; I do not have human passengers back there.)
However, I can tell you this set-up will not work with the doors off. I did try it once, driving in the driveway, slowly, and my "smarter" boy ended up on the ground trotting beside his door. Well, kind of trotting - he could not get all 4 feet down at once due to the shorter leash. Naturally I was just doing a test so basically saw him fall out; he was leaning out and just slipped. He only "trotted" a few feet because I stopped immediately, of course. No harm done, but it showed me that for my setup, there is no going around without back doors. Maybe some of the guys who had center tie-down, or tie-up, have been successful that way. For me, though, I run without front doors, but leave the back ones on for the dogs. It's much less cool, but as a former prosecutor, I've seen too many traffic accident photos involving dogs. I'm not risk free, I'll admit it - I imagine if I roll it, my dogs will be badly injured by hitting the roll bar - but I think they are in pretty good shape to sustain a non-rolling crash. Good luck solving your problem. If your dog is like mine, it's damn near impossible to leave the house without him.
Oh, and I do run it topless. Just not doorless. One boy does not much like the wind and simply lays in the center right behind the console, and the other one dons his doggles and hangs his head out for maximum wind effect.