7 pin connector in bed
 

https://cimg5.ibsrv.net/gimg/www.f15...51dbc03b81.jpg
Because I found a knockout that looks like it's made for this located at the left rear of the bed, below the bed light knockout. Also One of the installers pointed it out in his '15 F150 bed. I'm asking anyone who has installed a wire harness to connect the factory 7 pin plug at the bumper to an auxiliary plug in the bed. The hole is rectangular and all the pre-made harnesses have a round socket. Obviously I want to avoid putting holes in my bed if possible.

A quick Google search shows that it's a rectangular fitting with a round connector in the middle.

https://www.etrailer.com/Custom-Fit-...ins/41157.html

You are quick on the draw! My more extensive Google search yielded about a half dozen options for pre-made harnesses(including Hopkins) that all have the same or similar box connections. Three I opened the installation directions for show drilling a ROUND hole. It may be that the back side of the connector will fit through without drilling but I found no dimensions to lead me to conclude that. I'm thinking there may be, but have not found anything, that would fit tight in the hole with a backing plate and locking collar. That is why I asked for any previous experience with this.
https://cimg0.ibsrv.net/gimg/www.f15...52cc1b39a0.jpg

You can probably get a 7-pin plug in there if you carefully draw out the hole and then cut it with a Dremel/die grinder. The existing square would represent the top of the finished hole. You don't want to go too high or the top right ear of the receptacle will be off the edge of the flat surface. It would make for a very neat and well protected installation.
I'm not tracking with the backing plate and locking collar. I've done many in-bed plugs and just used sheet metal screws. In the case of an aluminum bed, use stainless screws.

Just FYI that hole is for the switch to turn on the LED Bed Lighting.

Adapter
 

If I didn't want to slice up the bed, I'd look for a flush mount 7+ pin connector that would fit the opening, and then have the mate to that plug going to a standard 7 pin.

Be careful with stainless. You'll get corrosion because of the dissimilar metals.

Ditto, Stainless and Aluminum react badly with one another and is a sure fire way to cause corrosion. Use treated screws, I think phosphate coated are the ones, or something along those lines, has a black coating on them. Is there a particular reason you want to do this instead of just using an extension from the bumper?

I'll do a little more homework, but I don't THINK stainless/alum. is an issue. Ferrous metal and alum don't play well together, but I think stainless is OK.

"Corrosion risks with aluminium and stainless steel in contact. ... Stainless steel fasteners in aluminium plates or sheets are normally considered safe, whereas aluminium rivets or bolts holding stainless steel parts together is an unwise combination, as there is a practical risk of corrosion"

--This didn't copy across as I wanted, but represents my understanding of stainless vs alum. I've always been under the impression that stainless fasteners in alum. plate is OK. That being said, the phosphated screws might be a better option.
** Now I'll say this, and it's a position I'll never back down from until my dying day...Ford needs to be kicked square in the ass for using aluminum in the first place!!