Final answer:
The Vehicle Identification Number (VIN) Decoder on DealerCONNECT is the resource that is very helpful for finding out detailed information about a vehicle's speaker system by using the unique VIN of the vehicle.
Step-by-step explanation:
The resource on DealerCONNECT that is mentioned as very helpful for discovering details about a vehicle's speaker system is the Vehicle Identification Number (VIN) Decoder. This tool can provide specific information about features and specifications of a vehicle, including the audio system and speakers. By inputting the VIN of the vehicle into the VIN Decoder, it would give a detailed breakdown of the car's features, which is useful for anyone needing to know the exact details of the vehicle's speaker system.