stock radio problems
 

I have a 2004 V6 Mustang. My radio works fine when I turn the key to accessory, but when I start the car, the radio is all static. Any ideas on what's wrong?

You may have a bad cable going from the radio to the antenna.
If the cable ( coax ) shielding goes bad at the base of the antenna inside the fender,
it can allow electrical interference from the engine to get into the cable and pass to the center conductor of the cable and then into the radio.
Cables can go bad due to water getting into the outer base of the antenna where the mast screws in.

hadn't thought of that. Thanks for the advice, I'll take a look at it this weekend. The only thing I've tried so far is pulling the fuse for about 30 seconds and then putting it back in, but that didn't do anything, and I didn't really expect it to since the radio works fine when the key is just on accesory.

What type static is it?
Is it a whine that sounds higher in pitch as you press the gas, ( alternator )
or a popping / crackeling sound that gets louder as you press the gas, ( spark / MSD )
There are filters for this type of interference but a mostly stock car should not have problems
in this area. If you added high performance engine electrics or an after market sound system
you can possibly invite this type of problem and there are a lot of tests needed to find the problem.
But if the car is stock and the problem just showed up it is probably just the antenna cable and to
test this as we did in the old days we pulled the radio and plugged in a test antenna.

All the electrics are stock. Except I did add a performance chip to the computer... Could that effect it? The static doesn't change when I accelerate or deccelerate, but it is the crackling/popping kind of static.

From all the options you have listed, the antenna cable sounds like my best bet to test.

:) YUP :)