Chosen Solution
I split my machine open to replace a non-functional battery and ran my putty knife through the trackpad cable. I ordered a replacement per the PN# on the cable I cut, 821-1904-A. This however did not work, the trackpad has no movement/input at all. What is more odd is I inadvertently I ordered an incorrect cable, 821-1610-A, and it works. However, due to the design of the cable, it’s too long and can’t be installed in the case as a solution. I have also ordered a whole new trackpad assembly, thinking perhaps there was some relationship between the trackpad and the cable, again using the recommended part 821-1904-A and matching for my machine, Retina 15” MBP (Late-2014) At this point it seems that there is either (a) a strange “in-betweener” cable that I need, or (b) there is some kind of system reset I need to do. I have already tried doing both the PRAM and SMC reset multiple times with the new trackpad assembly installed.
You may want to replace the full part MacBook Pro 15" Retina (Late 2013-Mid 2014) Trackpad
