Sounds like a job for a 2-input AND gate.
Case 1: When the radio is playing and you press a button, send the command to the radio.
Case 2: When the mp3 plays and you press a button, send the command to the mp3 module.
You'll need 8 gates (4 going to the radio, 4 going to the mp3 module). You'll need to be careful to match voltage levels, commands active high or active low, etc.
It is possible to make an AND gate from transistors and diodes. Or even from diodes and resistors.