I would assume it is the ribbon cable connecting trackpad to the mainboard.
I had the same problem and swapping that out (2dollars from ebay) fixed it.
As a remedy, when i applied pressure under the laptop at the top and middle of trackpad, it sometimes came back to life.
P.S it is a really easy swap to do it yourself. And when you do swap it, tape the new cable so it wont rub against the bottom cover anymore.
Replace the flex cable that connects the trackpad to the logic board compatible with the Early 2015 model A1502 MacBook Pro 13" Retina laptop. Part #821-00184.
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