We have a 4038, not sure of camera cable location differences, but on the 4038 the factory supplied cable lies behind the large green-painted cover bolted to the back of the cab, just beneath the rear window. On mine, I removed the 2 "shoulder bolts" (1 3 mm wrench I think) on the RH corner of that cover. It will still be bolted on the LH side...but that's OK. Gently pull the cover away from the cab & look behind that RH side. You should see a cable with a dust cover on it & a square fancy weather pack type plug. The plug matches the plug I have in photo below. The cable on the sprayer is also labeled "video", I believe.

But before you do all that, you need the "new" JD adapter cable that goes from that square weather pack to the JD Voyager camera type plug. That's the cable assy I have in the first photo.

After  you've located that plug behind the cover, then connect the 2 halves of the weatherpack together. It helps if you have a younger more flexible & nimble person to do this under your supervision. I didn't have that person when I hooked the cable...but I wished that I had. After connecting the weatherpacks, then you can hook up your extension cable to this harness. Now again, this harness is to fit the JD type Voyager cameras & not the Cab Cam or Ag Com connectors, though maybe JD carries that in their parts. I don't know. If not, you'll have to go to Ag Express.

Finally, I always mount cameras on magnets & then add a tether besides. You can see my camera located high up on the back of the boom. Hooking to that "video" cable behind the cab allows the camera to populate the GS3 screen when you slide into reverse, or when you lock the machine into "transport" using the new transport button located on the RH console armrest. Good luck.