Hi: as L.Pfaff mentioned reseating the trackpad cables are an option and the following link will show you how to get to them:
[guide|515]
However you will be needing to pull the plastic covers on trackpad to get to connections. The guide I listed above will show you how to remove the whole keyboard upper case. But once you remove battery you can check one of the cables. If that doesn't fix it I would replace the entire upper case that has keyboard and trackpad. Note that you need to see the case before it ships to make sure it doesn't have the missing areas at edge of palm rests.
As always practice good static control when working inside your machine.