I'm having the exact same symtoms... Left channel is totally out upon start up. Then slowly sputters back to life after 10-20 minutes. Also, when changing stations via the presets, a popping/snapping sound comes from the speakers... This also goes away over time???? I checked the AMP at the dealer by simply plugging in a rebuilt AMP they had in stock... No difference.

Crew

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94 W129
98 Audi A4
98 BMW R1100RT
95 Chev S-10 4x4 ZR2