Thanks for the info on the fuse. I just read my 2009 Liberty manual and it did say you could get a Jeep service tech to make the lighter socket hot. I figured it was something simple. I may still do that for other reasons but I've found out a better solution for powering the BrakeBuddy.
The place that is installing my tow-bar brackets, etc. really know their stuff. They've dinghyized a bunch of Liberty's. They pursued the issue with Jeep to try to understand why the battery needed to be disconnected to tow. They finally found someone at Jeep who knew the answer ... you do NOT need to disconnect the battery to tow. The only reason they tell you to disconnect the battery is to keep your accessories from draining it. In the Jeep Owner's Manual they should have at least state that so you could make an intelligent decision as to whether you wanted/needed to disconnect the battery.
What the RV pros, Temecula Valley RV, that I'm getting to do my work are doing is running a line from the MH's battery back to the Liberty's battery. My MH has a 7-pin connector and one of them is 12V hot feed so it's pretty simple to get the power back to the Jeep's battery. With their approach, the Liberty's battery can be left connected and the clock, radio presets, etc. don't evaporate. While the Liberty is drawing current to run ACC, the MH is replenishing it. I guess if you stop driving and leave the Liberty's ignition on ACC, you could eventually drain both batteries. But I think you would deserve it if you left the ignition on ACC for days when you weren't towing.
BTW, I did get a chipless, the-engine-won't-run key made. The local Jeep dealer had the blanks in stock. With my VIN and an ID, they were able to do it in a couple of minutes. Now I can do the towing setup without being worried about leaving a full-function key in the ignition. I think the $14 I paid is cheap insurance to keep someone from driving off in my Liberty or even me loosing the $250 chipped key. Plus, I now have to keys outside the Liberty to unlock the doors.