I can't figure out a source for my memory wire with out running it straight to my trucks battery

[Southers]

I have hooked up a boss audio system n can't figure out a good safe source to hook my yellow memory wire to with out running it straight to my battery it's in a 1988 Ford f250

 

[popatim]

Dark blue with Red stripe is the factory radio memory line.
If you don't have that then use the constant 12v Green/yellow.