I am currently struggling with this $#&! on my '14 Silverado LT. Getting the old one out and pulling the shoe string was easy and I thought it would be a 10-minute job (excluding removing the trim). Getting the new connector to feed through the inner firewall is a ****** because there is jute or some insulation that just will not yield to pulling. (There seem to be two holes: the outer one into which the grommet seats and then an inner wall that opens into the passenger compartment.) It is impossible to get the cable connector to feed past the jute and then pull at the 90degree angle that the shoe string is pulling on the cable connector. In spite of having smaller hands, taping off the connector string into a conical shape, and having removed the battery, I am cussing up a storm (in english and hungarian) that any a-hole that designed it should be drawn and quartered. My son thinks it funny. I not so much after struggling a couple of hours. Oh, wire pull lube is of no use either.
Not sure if removing the wheelwell liner would help: it seems that that would provide no to minimal additional access.
Just venting
UPDATE: after cooling down, i went back out and within a few minutes I got it fed through. I switched over to fishing line which has less drag than shoe laces. Perhaps that helped?