Old plug wires can get leaky: the high voltage can jump to near by grounds. You may hear it but seeing it is more likely. Try a tarp or two, and try it with the motor warm, should do it more hot. I've never seen a benz relay that you could see into. They are in metal or black plastic cans, so you could rear ticking but you'd never see the internal arcing. And it would be hard for a relay to sound like plug wire leakage. The two sounds are very different.