This past August, I rented a U-haul trailer and towed my '15 GT, when on the u-haul website, the trick is to pick the right conditions, the weight is critical, I picked the higher one as my build is rated for a 13,000 lbs. trailer
I had the same problem when I connected a u-haul car trailer, dealer replaced the right hand door lock button, that might have solved the issue, will find a trailer this coming weekend to verify.
I have the optional B&O in my XLT, like any good system, you have to work in the speakers, once they are worked, the sound is awesome for a factory system (I have had systems built before)