while i dont have the music box system in my truck... couple things to do.
first things first, don't try to force the card into the slot. you could bend the pins in the back of the slot. in fact, you may want to look into the slot to check if the pins were bent by the previous owner. if they are bent, then your going to have to figure out a way to straighten them. i would suggest you start with a tooth pick or some other non-conductive material. just dont break the pins, or the toothpick. if you have to use something metal, i would turn off the truck and pull the radio fuse (dont want to short anything out when you're monkeying with the pins)
make sure you are trying to put the card in right side up. it really should only go in one way, but i suppose if you force it, it may go in upside down.
i dont know if there is an "eject" button for the CF slot. i know on my camera, the CF card can only be ejected by pressing the button (it is a lever system that pushes the card off the pins). if there is an eject button, make sure it is fully pressed in before you insert the card... if there isn't an eject button, then you can probably ignore this suggestion
good luck, maybe someone with the music box system can chime in with some direct experience.