There are 2 wires that are "unpopulated" on all of the available wiring harness adapters in the US marketplace for the smart. In other words, there is just a blank spot in those spaces. One is for an "amplifier turn-on lead" with which the radio puts out 12V when it is on. This 12 volts can be used for applying power to an outboard amplifier, a small antenna signal booster, etc.
The other wire not present is for "illumination". The aftermarket radios will have an orange wire that needs to be connected to a wire in the smart radio wiring harness to allow dimming of the radio display at night.
To do a professional installation you need to connect the "amp turn-on" lead (blue/white) from the aftermarket radio to the 2 thin black wires. This connection will of necessity be made outside of the aftermarket radio wiring harness adapter, since there is no wire present in said adapter. If you connect the "power antenna" wire (blue) to these 2 thin black wires you will lose power to the subwoofer amplifier when listening to any source other than AM or FM. The 2 thin black wires are assigned thusly: one goes to a subwoofer amplifier turn-on relay, one goes to the small antenna signal booster. Since this car does not have a moving power antenna (UP/DOWN) there is no harm in having power at the antenna signal booster 100% of the time when the key is on.
Aftermarket radios have a blue wire which only is ON when you are listening to the radio. This was originally so that you could lower a power antenna when desired, for example at a car wash. When switching to another source, such as cassette (back in the day), the power antenna would retract into the fender, which emulated most of the factory setups.
The orange illumination wire from the aftermarket radio needs to be connected to the light grey/blue (light grey with a blue stripe) wire present in the factory smart wiring harness for the radio. Some of these smarts use violet/blue, but 14 out of the 15 I have done are light grey/blue.
Depending on the software and user-changeable settings of your aftermarket radio, this orange wire hookup allows your radio to dim when the headlights are on, which is important with a navigation radio because the display is really bright at night. Some aftermarket radios such as Kenwood now sync the dimming to the built-in Garmin nav system, so the radio display dims at sunset, since the Garmin nav will go to a "Night Mode" at sunset, which of course it knows what time that occurs every day because it is a GPS system. Therefore strictly speaking you don't need to connect the orange wire to the light grey/blue wire, but it is good installation practice to do so anyway, since you cannot predict at the time of installation what features/benefits you would lose by NOT connecting it. I have installed 15 GPS aftermarket radios in 15 2008 smarts so far, so this info comes from real-world experience.
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