Are there two yellow wires?
If yes, then the yellow wire next to a dark blue wire and a brown wire is the speaker left rear negative and the yellow wire next to an orange wire and a black wire is your switched twelve volt positive.
This should be comfirmed with a digital multimeter before you wire anything up.
If this doesn't work, then you have to use another switched source from elsewhere.
You might have GM's data bus, or something ruther, system with does not
have a 12 volt switch, if I remember correctly.