If you wire the D+ terminal up from behind the radio without a bulb or diode in line you will damage the alternator. What happens is when you turn off the ignition key the alternator is still charging until the engine RPM is zero. This can take 1 or more seconds. In that time the little exiter diodes are powering everything they are connected to ie in this case at least your radio. They can deliver approx. 10 Amps, that's including the current needed for the field winding. They will eventually burn out.
So fit a diode in the wire to D+ and Bob's your uncle.