Chosen Solution
After replacing the battery and trackpad per the iFixit tutorials, my Late 2013 15” MacBookPro won’t turn on. No fans, no lights, no nothing. Plugged into a charger, the charger light turns amber, and resetting the SMC while it is plugged in causes the charger light to go briefly from amber to green and back to amber (which makes me think the keyboard is connected correctly). Any suggestions on what else to check or how to debug this? All connectors removed seem seated correctly and none were damaged during the removal.
I had a similar issue after replacing my battery. I was able to fix it by removing the cable that connects the i/o board to the logic board, clean it out, and make sure it is seated correctly and snugly. After that cable was correct it booted right up.
