Vintage Pioneer KE-81ZBM-02 radio/cassette player gets power, but no "CODE" in display and no flashing anti-theft light.

Greetings. I have cleaned wheels and replaced the belts in the cassette player for the Pioneer KE-81ZBM radio/cassette unit in my 1988 BMW 735i. I think it is not the original unit, but has been in the car for many, many years, when my dad owned the car. I have the code. When I placed the unit back in the dash, it powers up, I hear the "click" when I turn it on, and the lights on the buttons illuminate. However, I do not see the the word "CODE" in the display and the anti-theft flashing light does not illuminate. Also, the cassette player does not want to accept a cassette. When I enter the code, I can enter all five numbers and then I hear an audible "beep." I am sure that I have made all the correct connections. I think I may have more than one problem. Thanks.