I changed the keyboard itself; you follow the instructions to step 35 (skipping 29 to 30, microphone removal) and then remove the keyboard ribbon cable connector to the trackpad (step 45). Maybe the most complicated step is ungluing the black backing stuck to the back of the keyboard and then there are almost a hundred tiny screws holding the keyboard to the frame, mostly only an eye strain. Got the keyboard through ebay (spanish keyboard layout).
Thank you, iMacs are not obvious. Would have been a real pain having to find out by myself.