We are going to set up a 2015 Silverado to flat tow and also have to disconnect the negative battery cable. There are 3 basic options. First, run a wire before the battery disconnect switch to power the brake system. Second, run a power wire from the tow vehicle to the brake system. Third, I have read where some people are using a battery jump start system that has a storage battery to power the brake system.
Of the three, I will probably run a separate power wire from the tow vehicle.