Hot take: I'm done with those 'one size fits all' laptop battery replacement videos
Tbh, I tried to swap the battery in my old Dell Latitude last week, and the guide I watched skipped the part about the tiny ribbon cable under the trackpad. Ngl, I tore it trying to force the assembly out. Anyone know a good source for E5540 parts that won't cost more than the laptop?
My E5540 needed a new keyboard last year and I found a whole parts lot on eBay for like 30 bucks. I mean, @willow_garcia is right about those cables, you really gotta go slow with a plastic spudger. For that specific ribbon, maybe check the Dell part number on the cable itself and search that.
Did you ever try checking if any of those generic cable replacements from China actually fit? I had a similar situation with an old HP where Dell ribbon cables from a parts lot saved me a trip to the electronics recycler. @blairj55's tip about searching the part number is spot on, those tiny numbers on the cable are usually a lifesaver for finding the exact match.
Ribbon cables are the worst part of any repair. My friend bricked a perfectly good tablet trying to replace its screen. Sometimes it feels like you need a surgery degree for this stuff.