Car Stereos: Head unit will not turn on but has power., digital multimeter, power fuse

Question
I have had a working dual head unit for about a year now.  Yesterday i attempted to install a new alpine unit in my 1990 cavalier.  By accident i switched the ignition and power wire to the unit; after realizing my mistake i had correct wiring and unit would not turn on.  I tried installing my old unit and that would not turn on either.  The dual unit had a cd in it and when i turned the key it, the cd ejected, but the unit would not turn on. I tried using two separate grounds, and fuse is still in tact.  What could be wrong?

Answer
Hi Joe,

I'd suggest going through and checking the fuses again.  Remember that most vehicles have at least two fuses in the fuse box that will supply radio power:  one for the key-switched power wire, and another for the constant power connection.  I'm betting that one of these fuses is blown.  Often, the constant power fuse isn't identified as a radio fuse--it might share a circuit with the dome lights, locks, horn or cigarette lighter.  

If that doesn't work out, then you'll probably want to do some testing with a digital multimeter.  In your car, the factory constant power wire should be orange; the accessory power wire should be yellow.  There's also a yellow speaker wire, so you'll want to make sure they aren't mixed up.  You can use the multimeter to verify that you have voltage on the orange wire at all times, and on the yellow wire when the key is in the "accessory" position.  

Hope this helps!

Brian