1) Establish that you have voltage at the battery. As other accessories
powered by the same battery are reported to be working, that is a safe
assumption in your case.

2) Test for voltage where the wires terminate at the stereo. If you
have sufficient voltage and a complete circuit to that point, the
problem is then either an internal problem in the radio or (possibly)
lack of proper grounding. Do this test even if you are using two short
pieces of wire to connect directly to the battery for a test. Don't
just assume you're getting power because a
3 ft wire is connected directly to the battery- there could me an
internal problem with the wire. If you're using cheap, solid
"automotive wire" vs. proper, stranded "marine cable" the chance of an
internal problem goes up a lot- particularly with an older wire.

3) If you have power at the battery but no or insufficient voltage
where the wires terminate at the radio, you have a dirty, loose, or
improper connection between the radio and the battery. If this is the
case, start at the battery once again and trace the wires to any buss
bars, circuit breakers, etc that may be between the battery and the
radio. Test for voltage. When you find a connection without power or
proper voltage, look carefully at that connection, and look again at
the connection immediately preceding it in the circuit.