Hi guys, I understand I'm a bit late to this particular thread but I fixed this issue on one of my customers MacBook airs recently and have fixed it a few times before, it is almost always an issue with the cable that goes from the trackpad to the logic board, take this cable out both sides and ensure there is no corrosion or burn marks, on the recent one I did there was slight liquid damage on the cable but the connector was pretty clean.
Just thought I'd post this in here so if people come to this issue in the future this will hopefully save them the headache!