Some other observations:

The hum is the same loudness regardless of radio volume setting. Same in either radio or cassette mode.

Hum is present without turning key on (i.e. engine off) or car moving if radio on and lights on.

Hum produced when parking lights or headlights on, but not when rotating light switch left for left only or right only parking lights. (not sure what those are really called)

When radio volume turned down, hum seems really loud, but when volume up not very noticeable with radio audio. Especially with highway noise.

Could the circuit that detects burned out bulbs be responsible? Anyone know how it works?

__________________
Joe Myers

'83 300D (light blue)
'82 300TD (off white - parked)
'87 300SDL (silver)

'75 300D (black - sold)
'76 300D (beige - totaled by pizza delivery car)
'82 300D (metalic blue) sold