Hi,
Can anyone tell me exactly what this circuit is? I would like
to tap into for a two radio drawing 15 amps.

 

Fuse 24 in cab fuse box(SJB) is Key on Power, so is most likely Powered by Ignition switch fuse which is 50amp, in engine fuse box
It shows Audio system only

And fuse 24 should be 20 amp so shouldn't be an issue using 15amp draw
Be sure to use wire size appropriate for 12volt 20 amp circuit, smaller wire = higher amp draw because it heats up