ok need to ask WHY WOULD YOU TAP INTO A LINE WITHOUT KNOWING WHATS ON IT ?

power wire off the battery!
then you know what is on it and where it goes

 

Considering it draws VERY little current (it's basically using your car battery as a button cell battery to store memory) I don't see any reason not to use the cig lighter. In almost every car I've seen it just goes straight from the cig lighter to the fuse box to the battery.

 

my cig lighter set up went funny and i was not using it that way
i had my ipod getting power form it

 

Most decks draw their currrent from the constant wire, so the easiest fix is the battery, with a 30 amp fuse of course...10 ga should do for an 80 watt output deck.