Car audio question

Typical for DIN head units is red for tuner power hot at switch on, and yellow for memory hot at all times. Sometimes there's a orange for illumination too. A lot of aftermarket head units have a little box inline with these wires going through it. Its a fuse box. Maybe open it and find blown or missing fuse(s).
I've picked up a few used units over the years and found all those manuals available in .pdf online.