I wrote recently that the designers of the Sonic were a little short-sighted (I may have used the term moronic) when it came to designing the car. When I was towing it, I was killing the battery because the car had to be turned on for my auxiliary braking system to work. After 7 hours on the road, there wasn't much juice left in the battery.
Well, it turns out that there is a second adaptor which bypasses the battery and allows the car to be towed. It would have been nice if someone had mentioned that to me at the dealership, but it just goes to prove the old adage... when in doubt, RTFM (Read the F***ing Manual).