This whole thing reminds me of when I bought an Audi Q7 for my wife back in 2014. I got the thing home before I realized it did not have a receiver hitch on it. I called the Audi dealer and they told me it would cost $3400 to add the receiver hitch. I thought that was a bit high, and looked for other hitches, and found they could be had for $150. BUT because the car is all computer controlled with LED lighting, you can't just go splicing wires to control your trailer's electronic lights. There was a trailer controller module that had to be added to the car, to control the trailer lights (and brakes). But I quickly learned that this module was only sold as part of the full hitch kit. The same one that Audi was wanting to install for $3400. I ended up getting the kit from Amazon for $950, as it was a pretty common kit, being used on the Audi Q7, Porche Cayenne, and VW Tuareg. So, I got the kit and removed the bumper skirt and rear bumper, and replaced it with the new hitch, and installed the module and put it all back together. It was then that I learned that the car did not recognize the module, and that it needed to be turned on in the car's computer. I tried finding a shop that could do this for me, but none could help (or would not help). I ended up having to purchase the Ross VagCom system for $350, and do the re-programming myself using my notebook computer and their software and interface cable. I had to activate the module, and then change many other settings, so that when a trailer harness was connected, it disabled the backup sensors, stiffened the rear suspension, widened the range for the radar adaptive cruise control, widened the view of the blind spot detection, adjusted the shift points on the transmission, and adjusted the anti-lock braking system for a heavier load. It was insane... The lesson I learned was that on these complex machines, DIY electronic hacks can be more trouble than they are worth.
When I got my RT, and wanted to add a flashing brake light, I started researching and learned that I would encounter many of the same issues as with the Audi. I ultimately decided to go with the DENALI SmartCAN system, and their B6 tail light, and added a pair of DR1 driving lights along with it, and pre-wired for a 2nd set of future lights and an upgraded horn (probably a DENALI SoundBomb).
So, getting back on topic of your top box lighting... Your only option may be to get a BMW kit that works as a plug an play solution. Unfortunately, I don't know if these are available, or at what cost.
I have always said BMW is an abbreviation for a German phrase that roughly translates to "we will nickel and dime you into poverty", but I still think it's worth it for the quality and performance of the RT.
I will also do some googling, and if I find anything on a harness, I will post it back here, if you have still not found a solution.
-Dana