Adding 2 Czech Korun:
Cheap switching power supplies work best in the "circular file".
Proximity and relative angle always play a part. Wires at right angles to each other tend to induce less. Parallel and close wires are a recipe for induction.
Another common wiring error is to use wires that are too long and keep the excess coiled up. If you must coil excess wire use a "figure eight" pattern.. Better yet, shorten the wire.
The use of a "star point ground" as mentioned earlier is very good, but the length of the wires that are not common (from the star point to the instruments) is best if short. The ground potential between instruments is then less.
Metal instrument panels will provide another ground reference to some instruments, and poor wiring elsewhere might give you entertainment you weren't looking for. For example old Cambridge CVA audio modules (I seem to remember suffix of 10) were a problem in metal panels. Using the push to talk indicated lift. Dave Ellis described that fix (cut a trace on the circuit board) decades ago.
Many but not all aircraft radios have balanced microphone inputs. Essentially, the microphone has a positive, a negative and a ground connection. The radio cancels out the noise induced on the + and - wires, so connecting all three wires from the microphone to the radio helps keep noise down in the microphone circuit.
Jim