All goes dead when I turn the key

That's typical of a bad connection. To find it, turn the head lights on so everything stays dead, then use a test light to see where the voltage is lost. Start with the ground clip on the battery negative bolt and the probe on the positive post. Move the ground clip to the body, then move the probe to the next point down the line. If there's a smaller red positive wire going to an under-hood fuse box, test there. Otherwise go right to the large terminal on the starter. The testing has to be done while the problem is occurring. You're looking for the first place where the test light doesn't light up.