Hi, There is a security plug in wire on the passenger side of the radio, if you do not plug this back in to the radio after you reset the codes you will not have this issue again. ( I think.)

It was several years ago that I had this happen to me and my MB dealer did reset it for me. I have replaced the radio with a CD-AM- FM since. See if you can find a vintage radio repair shop. Yellow pages is a big help. Best of luck. Jack.