Was anything else done to the vehicle while you were in for service?
Sounds crazy, but twice I have had ML's with radios that will not turn off when ign. key is removed , and overhead control panels that won't work -removed sun visors and found pinched/punctured wires.
And to sound even crazier: I found it because the radio would not shut off until i opened the mirror cover on the pass. side visor.
But your Harmonic Balancer repair has nothing to do with the radio.

No, supposedly there is no way to get a radio code for you vehicle except in a service Department. However, a few times Roadside Assistance has been known to give out radio codes as long as you can convince them over the phone - just depends on who you talk to. worth a try calling the number on your drivers visor.