I found this thread just in time, as I just re-installed the head/amp/tuner in my 92 300TE. Powered it up, entered the code, and got an error E2.

According to one of the earlier posts in this thread, some of the potential reasons why the system throws an E2 is due to no power to the RECEIVER UNIT (next to the rear washer fluid bottle in a 300TE).

Don't just go out and grab your voltmeter, first look at fuse C. If C is missing or blown (my mechanic pulled mine to deal with a locking controller issue), you won't have power back there, hence the E2 code.

Putting the fuse back in C fixed it all for me. And there was much rejoicing.

Chris