You're talking about a circuit fused at 8 amps that has a clock (negligible current), an interior light (~1 amp), a trunk light (~1 amp), the power antenna (~3 amps), the rear cabin light (~1 amp) on it. The Mercedes electronic radios used the always-hot wire just for the station memory, which drew little current. Modern radios use it to power the entire radio. If you do it your way, be prepared to blow fuses.