'99 Sable Radio/clock no power - need advice

Looking for suggestions on how to troubleshoot wiring to '99 Sable radio unit with heater controls and 6 disc cd changer for my mother-in-law.

The fuse is fine.

I replaced the entire unit with a used one from Ebay and got the same result - radio/clock/CD still does not work - this part of unit doesnt even get power. The heater controls are getting power and function properly.

I didnt notice anything visually wrong with the wiring harness connector at the unit when I pulled from the dash. I am assuming it is in wiring since the original and used replacement radio behave the same way.

I looked through threads from all of last year and it seems a couple other people are having erratic trouble with radio/clock as well.

Any advice or encouragement is appreciated.