I guessing that your player is set to MTP mode, maybe due to the fact that you could be using DRM-crippled audio book files. It needs to be in MSC mode for your car’s stereo to be able to see it. If it is in MTP mode, the stereo cannot see it, and may be falling back to the card slot; in your case with the SlotRadio card in it.
If you are not using DRM audio book files and the player is indeed in MTP mode, then I’d suggest deleting them (or transferring back to your computer) from the player, switching the player to MSC (in Settings > System Settings > USB Mode) and then moving the audio book files back to the player.
Now see if your stereo unit will read from the internal memory of the player. If it still defaults back to the external memory card, un-plug the player from the stereo, remove the SlotRadio card, and plug the player back in. It should read it now.
If you are using DRM files and can’t switch to MSC mode, or the above doesn’t work for some reason, try the AUX IN port from your headphone jack. It’s not the ideal way as you’ll have to control things from the player instead of your car stereo unit’s controls, but it should work.