Cadillac Seville / Cadillac Eldorado Forum Discussion, 1980 Seville, is this the right forum? in Past Cadillac Vehicle Discussion; Is this the best forum for parts and info for a 1980 Seville?
I just got one, for free.
Your best bet is look around at used auto parts dealers (Junkyards). Some specialize in Cadillac parts. That is especially true about the cruise lever and a factory stereo. IIRC, window regulators use a perforated nylon belt. That might be tougher to find.
Rear air shocks should be available from any parts store (but they won't be AC Delco). They might need to be adapted for the Caddy air lines.