Car stereo units have had isolated (non-earthed) audio spkr connections since the early 80's. This reason for this is twofold. One is to stop earth loops for noise etc, but mainly because the output stages in the radio are isolated from the spkr case which is normally connected to battery eth.
The units then need to have both the power and return wires run to the spkr or amp.
You can install an isolation unit between the spkr and the amp which then allows the spkrs to be connected to battery negative, but this can lead to problems. Easy to check. Connect multimeter from spkr neg wire to chassis. Is open reading then isolated. If short, not
mahrie
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