It did not go quite as smoothly as I had hoped because the space behind my stereo was not directly under the AM/FM antenna as I had thought. It was a foot or so away with a roof truss in between. Try as I could I just could not get a fish tape through the same hole as the antenna wire nor could I pull the antenna wire far enough through to attach the satellite antenna wire to it. I then thought that since it was so difficult I would probably get the plug end stuck part way through anyway so I abandoned that idea.
I then decided to drill a hole through the ceiling inside the adjacent cupboard which is directly under the AM/FM antenna. I inserted a tidy sleeve in the new hole and brought my satellite antenna down through it and then through the cabinet sidewall into the space behind my stereo.
I then filed a slot for the satellite antenna from the AM/FM antenna hole on the roof to make room for the satellite antenna wire. I then gooped the area around the hole with Dicor, re-installed the AM/FM antenna and gooped around the satellite antenna and over the wire.
Here is the AM/FM antenna cleaned of old Dicor showing the screws partly removed. Note they are on an angle presumably to give added strength in the thin chip board roofing under the rubber liner.
This is the hole you work through.
The finished exterior installation.
And finally the finished interior installation.