The radios that come up with dashes ---- can not be reset with a code. They will need to go to the dealer to be reset. These radios were installed from 1986 to about 1990. These radios do not go into code mode unless their internal battery is dead. Be sure to replace it first and then there is no problem disconnecting the radio from the cars battery.
Bosch Master, ASE Master, L1
33 years MB technician