Just in case its a Grundig, there is another potential problem. This assumes its a 450 model. When the battery is disconnected, it puts the radio into the SAFE mode. You cannot get it back on without the 4 digit reset code. Most often, the four digit code was on a card in the pullout tray just below the radio. Look carefully for it. If not there, go through your documentation, if any, that came with the car. See if there is a code somewhere in there. If there is no code and no card, you may end up having to get a Sony after all. The Grundig won't do you any good without the code and it may take a call to Germany to get the answer of the correct code. If you do find the code, use this process to reset the radio: Press the CD button, then use the four buttons below the radio, pressing the one on the far left first until the number on the readout area is the same as your first code number. Use same procedure for each of the next three numbers, then press the CD button again when you have the correct code showing. Your radio should immediately start to play. If you don't have a Grundig, you've wasted a lot of time reading this. Sorry.
Ron Gaus, asmartcar.com