What trim level? Do you have the additional camera for when the tailgate is down?
Mine is also a Lariat with 360 camera. No extra camera on top of tailgate for when tailgate is down. Curious if your KR with double tailgate camera will act different.
Have you tried plugging in a camera and seeing if your backup camera works? With it unplugged, you'll get a blue screen, of course, and all your other camera views should continue to work (since yours is a '24. It is the '23 that all cameras go dead when the backup camera is unplugged).
Since you have an HBE, presumably you have a flatbed tray installed, no bed, and so not even the option of attaching a tailgate, but hopefully you have the tailgate handy and can do the experiment below if you have not already.
First, how many tailgate camera connectors do you have? one or two? Not sure if Ford somehow ties both cameras into one wire or keeps them separate, but likely there are two camera connectors and you currently have both filled with the blanks provided in the Tailgate Removal Kit. You can experiment easy enough by rolling your tailgate on a dolly under your truck, eave your 'tailgate removal harness' plugged in, but take the blanks out of the (presumably) two camera connectors. Plug in the first tailgate camera into the first backup camera connector. Start the truck and put it in reverse (with brake applied so you don't move) and see if you still have blue screen or camera view (pointing in a weird direction since tailgate is laying on a dolly). If still blue screen, move first camera plug into the second connector and retry. If nothing, then unplug the first camera and take the second camera plug into the first connector. If nothing, second camera plug into the second connector. #1into#1, #1into#2, #2into#1, #2into#2. You only have four combinations. Likely one of them will give you your backup camera view.
Note, there is a chance the two camera connectors are slightly different shapes so only the corresponding camera plug will fit into each camera connector. Which brings your experiment options from four down to two.
In any case, if one of them successfully gives you a backup camera view, you'll want to determine which camera lens is working. Easy way to do that is, once one of them is working, put a piece of tape over one of the two camera lenses to see if the view is blocked (or simply observe what is showing up on the camera view compared to the tailgate orientation as the regular backup camera view will be different from the second tailgate-down camera on the top of the tailgate).
There may be a fifth option just to try for fun which is plugging in both camera views and see what happens. That is actually a reasonable shortcut if you prefer. Plug in both camera plugs to both camera connectors and try it. If you get successful backup camera view, unplug one of the two and see if camera view remains or goes away. If it goes away, pull current connection and reconnect other camera.
Once one is working, would also be fun to see if you plug that same camera into the other camera connector if it works. I suspect that only one of your two camera connectors will work (depending on if the truck things your tailgate is up or down), but that either camera will successfully feed a view to that plug because I believe the same camera part number is used to feed the two views.
If you sold the tailgate along with the bed of your truck, then there are fewer options. You'd have to buy a camera, housing, and cable and try. But be warned that many of the knockoff cameras, with the same part number, don't seem to work. I tried an ebay one and it didn't work, but the one out of my tailgate worked as did buying a replacement of that part number direct from Ford.
So, give things a try, gather some data, and share that data. Then we can give recommendation from there.
