Even if the dash is still in place into the car, it's not a big challenge.
My Demon original harness was abused, modified/cut/welded and nearly out of service.
I found a kind member here on FABO that sold to me another used dash wiring harness, out of a 71 Demon too.
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Replacing the harness has been very simple.
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I just had to remove the cluster and after, it's very simple as there is plenty of room for the hands.
It's better to be a little bit supple to dive under the dash for the bulkhead connector, the brake pedal connector and screwing the fusebox :)
But it's really not a tricky job. I made the job in an hour