This is a diagram belonging to Series 4 trucks, but it's principally the same through Series 11, on the LUV's at least.

As they seem to have done with many upgrades and options, Isuzu opted to keep it simple and splice things into an existing circuit, rather than dedicate new wires.
In the case of the radio, the
windshield wiper motor circuit was their choice.
Also connected to this circuit are the cigarette lighter and the windshield washer tank motor.
All of these lead to the same
15a fuse located in the fuse box. Ser 11 had a smaller box than Ser 4, so don't trust the above diagram to locate the correct fuse.
If your wipers work and your radio doesn't, the problem is between the splice and the radio.