Starting in the late 90's, GM started incorporating the door chime and other warning chimes into the radio, and used the speakers for the chime instead of a seperate warning module. For trucks it hit in 03, so an adapter goes inline between the stock radio plug, and the aftermarket radio. Also the 03+ radios have no power feed wire to them, but instead use a signal from the class 2 data bus to turn off and on, so the adapter also turns the radio off and on. The problem with the adapter is, it doesn't listen to GM data bus messages when the scan/flash tool tells everything on the databus to go quiet so it can communicate with a specific module while the others go dormant while the quiet signal is being sent. So if it doesn't go dormant during the quiet signal, then the scan/flash tool cannot communicate, and many times will think the module is locked when in reality it cannot communicate because the databus is being occupied by a rogue module(the aftermarket one that doesn't listen).
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Originally Posted by bman
