The buzz is electrical noise. First find the source of the electrical noise. Is your door light an: LED or Fluorescent bulb? Fluorescent bulbs are notorious for being electrically noisy. Certain LED bulbs starting to show up in the industry are also noisy. So first is to make sure this bulb is a filament style; if it isn't then replace the bulb with a standard bulb (or filament bulb) - might be the fix right there.
Otherwise, the issue is likely in the power converter (the box converting AC to DC) likely part of your power panel has insufficient filtering for high electrical loads. The fix, whether sharing a ground wire or a noisy converter, is the same. You need more electrical filtering and noise suppression for the radio power. So you (or your favorite nerd) should pull the stereo and install more filtering on the power wire. Either install a very expensive and heavy power filter (or) install 3 or 4 basic 12V power filters in series with the radio power wires. This will reduce the noise to a nearly imperceptible level. Use caution and be mindful to disconnect the 12V power making any electrical modifications.
Have fun - electricity is cool, sparks are not cool.