Pull the fuse box down and you should be able to look at the connection point with a flash light. If the car is cool, you can touch the pipe and feel if any coolant has been dripping from the plastic clip connection area. It's an easier view if you pull the glove box completely out.Did you find that the leaking o ring was causing a significant leak, or just dripping coolant onto the carpet? After experiencing a CEM on the fritz, I just discovered that my carpet has got some coolant on it. It isn't flooded and that makes me think o-ring vs. heater core.
Mine was just dripping, maybe a few drops with every drive. I want to say about 2 weeks before discovering the actual problem, I thought the coolant level had dropped a few millimeters in the expansion tank, but I ignored this since I rarely park on a level surface.
I didn't mention this before, but my first attempt at replacing the O ring felt like it went flawlessly. Pipes fit together, clip fit on, topped up with coolant, and it practically poured out at the O ring area. Turns out I had a very slight pinch in the O ring when putting the pipes together. Luckily I planned ahead and had an extra O ring, but I don't even feel like the O ring is necessary with the hose piece I put on there.