Good start.

An easy test to start with would be to unplug all 4 oxygen sensors and pop in another fuse.
I haven't seen it but is is possible a heater has shorted. If it still does it, your stuck with hunting down anything else on that fuse and tracing the harness around.

I think its more likely one of the harnesses is touching the exhaust and melted or something else and chaffed though.

Good luck on the hunt, shorts can be a pain to chase down.
Keep us updated on the progress.