I just recently purchased a 1991 300E and i love it. Its a great car, but anyone will tell you that

Anyways i'm having a lot of trouble with the car stereo. I don't know if the battery was disconnected or it was taken out but for some reason when i turn the radio on, the screen tells me to "Wait." I read the manual and it says that you must wait 15 mins with the key in ignition possition 1 and then it will say "Code" so I can enter the code, but it just continues to say wait.

What do i do to fix this?
please help