I didn't save the link, but I was searching for this info recently.
(My 97 has power on the cig lighter 12v all the time, my 2012 doesn't).
In the fuse block (inside the car) there are some (unused) contact locations that have 12v with the key off, according to what I've read.
Use a multimeter and see if you can find those.
The instructions I found involved a jumper with an inline fuse from the fuse location for the 12v outlet to one of the 'always on' contacts.
A better solution (if you have a tool to remove the connectors in the fuse block) is to move the wire to a different location in the fuse block.
Spend some time searching for 'vw constant power to 12v outlet' and you'll find info and a pictures, from tdiclub, vwvortex and youtube.
Different methods.
It's on my 'to do' list once warmer weather arrives.