The accessory socket is dead with the car off, and you want it off to change the battery. A 9 volt battery connected directly to the battery cables might work. People have done what you want to do, so let's wait for one of them to post here.
2013 "Brilliant Silver" SV with Premium Package and no QC, and 2009 Vectrix VX-1 with 18 Leaf cells.
The most offensive, tasteless phrase in use here is "Pulled the trigger." I no longer respond to posts that use it.