Power, when the KEY IS ON, and 0 when the key is off. A keyed power source. Any, KEY'd ON source. Like, the keyed on wire to your head unit. Same same....
Meter, set to DCvolts. Hot to the fuse or wire or whatever, Ground to metal.
Is there 12v with the key off?
Yes- Find another source/fuse on fuse block
No- Turn key on,......... is there 12v with key on?
No- Find another source/fuse on fuse block
Yes- Tap into it. Enjoy