If you have connected it to the raido, it could be a fault in the radio.
Check all you grounds, make sure that your arn't causing a short. A drop in voltage on a power rail is ususlly becuase you are drawing to much current on a very small wire. but if nothing is connected to the ends of the wires (ie the head unit wires aren;t connected to anything) I Can only think that either you have shorted something as you were working, and the fuse didn't blow, but melted some wires! This will reduce the amount of current a wire can cary, making it act like a resister. Or you have a dodgy, rusted connection somewhere.