If you have a shared ground problem you would be seeing allot more then just your ign and radio going out. It would probably include dash, head lights, etc. Also, you would think there would be a fuse blowing, but it's possible a fuse was never installed. Last, the ign sys, though the ground is common throughout the car, usually runs a good solid ground to the eng or chassis seperate from anything under your dash, so that only leave 12V as the common problem.