I am trying to figure out why my rear defroster blows a fuse (red 16A) every time I power it up. At first I figured it must be a bad relay, so I replaced it with one from my parts car, same problem. Could be both relays are bad, is there a way to test them?

Next I checked continuity across the defrost grid on the window, checks out good.

Next I tried the relay without the positive lead to the rear window connected, this time the switch lights up and no blown fuse. On off, on off no problem of course other than I am not giving juice to the rear window.

Should I next guess that the line from the relay to the back window is grounded somewhere?

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1972 280SE 4.5