No luck? Hm. Dont lose hope yet. Can you manipulate anything to make the problem occur anytime?
I'd check for power behind the radio next, with a test light. Make sure you have the radio code, and then pull the radio. Find the power wire, IIRC its the big red one, and check for power there. Test lights are ~5 dollars, and are very easy to use. You ground the clip on the chassis somewhere, and if theres power at the wire you're testing, a light will light, hence the name test light.
If you have intermittent power, then you know the radio isnt at fault, and that the ground is likely good. If you have steady power, then test the ground. In order to do this, you need to put the test-light's clip on power. Then, find the ground wire and poke it. If you have a good ground, the test light will light up. If not, then the ground is at fault