So much negative has been written on the internet about this topic, I can't speak for later C4's but on the early ones it's actually not that difficult to replace. Forget about pulling the dash or carpet out all that's nonsense. Everything can be accessed from below, just pull the passenger seat out put a blanket down, find the best way to pull yourself in and out and get used to it. Pull the panel, the one with the light, drop the computer and disconnect it, remove and electrical vent motor also disconnect the threaded rod goes to it. Remove the plumbing. Pull all the cables out of the way. Remove the metal bracket. Then with patience all the 7mm screws that hold the cover together. Wiggle the cover loose and out. Then finally you can remove the core again 7mm screws. There was a vacuum line had to come out also earlier. It took me a couple hours only because of the online confusion I ignored reading any instructions on how to, so I had to spend some time in/out looking for tools. I did have a major problem when I went to put the new heater core in it didn't fit. At least I had some practice putting the cover on/off several times until I pulled the new heater core out and noticed it was too wide and that's why the cover never completely closed properly. No big deal I'll bring the original with me and match it up at the auto parts store.
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