Depends on the age of the car, on mine I ran a new cable from the battery to setup a constant and ignition "busbar". So when ever I have to add to it in the future I just branch off that, when I get some more cash flow I'll be installing something like this...
Oh by the way, those fuse adaptors are a good idea on newer cars where the wiring is a little stronger, but not for old cars. Imagine having an original 10amp fuse then adapting another 10amp onto that.....20amps on 3mm or less wire, not such a good idea.