You buy an aftermarket radio wire harness adapter for your manufacturer/model/year vehicle, you compare the diagrams for the adapter and the wire harness from the aftermarket radio and splice them together according to the diagram.
I highly, highly recommend against cutting the vehicle wire harness and splicing the wires directly to the radio wire harness. Not only is there just not enough room to work with to do that, any mistake or problem you have, could damage a very expensive wire harness that is extraordinarily difficult to replace. Do like everyone else and use the adapter harness, they only cost $15 or there abouts.
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