OK guys, reporting back. See what you can make of this!
Decided to try a noise suppressor first. Wired it as instructed and the first thing I noted was that the stereo was nonw 'live' even with ignition switched off. Presumably, the noise suppressor connects the two live wires internally (switched live and battrey live). Anyway, I turned on the engine and the interference continued, the supressor made no difference.
I then started to disconnect it; as I did so there was a quick flash as I reconnected the switched live wires. At this point the suppressor was still connected to earth and the live battrey feed.
I then removed the suppressor completely and with the oem wiring to the stereo reconnected found that the unit was dead. The live feeds were not live anymore.
I checked both the radio fuse in the fuse box and the in line fuse from the stereo and both were fine. Stereo still dead.
Finally, I notice that the seatbelt alarm circuit was not beeping at me with ignition on.
So, what on earth happened and why is the live feed to my stereo dead and the belt alarm also dead??
ps I quickly wired the stereo into my old Mopar and it worked fine, so what happened with the Buick?
Thanks for reading if you got this far!