1992 Mercedes 190e radio does not work just reads hold on the screen?

If it reads "HOLD" it means the code that unlocks the radio has been entered incorrectly more that 2 times. Simply wait for about 15 minutes after the last improper code input and it should then read "CODE" (the radio must remain on for those 15 minutes). If you don't have the code, any Mercedes dealership will give it to you free. Just call and ask. Be sure to have the VIN number on hand.