For the benefit of those searching in the future for Magnadyne 99xx information I'll sum up my story.
I talked to several local radio people and two Sirius customer service agents and consistently received conflicting information on the compatibility of some of the older Sirius components with a newer radio.
I finally talked to a Magnadyne rep who was both informative and candid.
The issues of the older 99xx series Magnadynes that were installed regularly in motorhomes may involve broken cold solder points, CPU gone bad or one of the printed circuit boards cracked. They no longer repair these units and do not recommend going else where as they're just too old, brittle and you're better off to buy a new radio regardless of the make.
In my case, the Sirius tuner (M9SIR), functions fine however despite the newer radio heads still having the same Sirius connection at the back the Sirius tuner will NOT work with it. It is not backwards compatible. When Sirius merged with XM the new equipment changed.
So what have I done?
I bought the 1.5 to Single DIN adapter plate from Magnadyne, which is plug and play, and then bought a Single DIN Sony SiriusXM Ready radio head for $100 CDN (probably around $30.25 US today
Now that left me without a tuner. I then bought a new SiriusSM SXV300 vehicle tuner on Amazon for $37 free shipping. The new radio will connect to this new tuner which I will install where the older one is under the dash on the driver's side of the coach. The connector is a mini DIN. I can then use my existing Sirius antenna which terminates where the new tuner will be mounted (in place of the old one).
The Magnadyne wiring harness uses the same color coding as the new Sony so the connections will be straight forward.
So there ya have it.....needless to say the new quick and dirty replacement radio has more features than I'll ever need....BT, USB, Pandora Music ad nauseum.