1991 Mustang GT Question about power wire to radio

There will be a yellow wire for constant 12+ and a red wire for switched 12+. I would double check all the fuses. If they are fine then it would be easiest to just run a new constant and switched than trying to tear the whole car apart to find the problem; if its not draining your battery. There are plenty of constant and switched wires you can splice into under the dash. Just make sure you add a fuse depending on what wires you decide to splice into. And definately solder everything. Hope this helps.