I just completed this installation this weekend. I had no problem plugging the factory male harness connectors into the supplied Shelby harness, but I clicked these together before trying to plug the new harness back into the factory amp. They do take a very firm push to seat however.
Here's where I ran into my biggest problem, I simply couldn't get the Shelby harness plugs to line up and plug into the factory amp. The problem is that the factory amp is high above the driver's kick panel on my '13 Premium and the plug sockets face forward away from you, so its a total Brail job! After about 30+ minutes of cussing, I finally got the first plug to go. The second one was much easier using the first as a point of reference. Again I found you had to push these in surprisingly hard to get them to click home.
I ran into the same problem with the fuse. The one they supplied was an ATO regular sized 30A fuse and the power harness that connects to the battery was designed to accept the smaller mini blade fuses. I had a 20A mini on hand so used that instead. I called Kicker to make sure of the amperage and they told me to use a 25A mini and offered to send one in the mail to me. I'll pick up a 25A as a spare in case the 20A one I used blows. Kicker told me the 20A will probably work fine.
I struggled a little bit routing the harness over from the drivers to the passenger side, there's not much room behind the dash and the suggested route in the instructions is pretty much the only one that will work. I had to stick a long screwdriver through there and push back all the insulation to clear a path.
I did not have a problem with the ground wire length, but there certainly wasn't any extra length available.
I installed the Shelby Kicker sub in the trunk while I was at it and had no real problems with that. I had previously installed the four 6x8 and 8" Shelby/Kicker speakers, so now I have the complete upgrade. Everything is working fine and it all sounds great. Other than the compact sub tucked into the right hand corner of the trunk, everything looks pretty much stock.
2013 GT Premium Brembo
MGW, Blow Fish, AM Hood Struts
Tinted, Borla Touring AB, ProCal Tune
Full Shelby/Kicker Audio Upgrade