It won’t work. Your car’s audio system can/will only see/read the internal memory of the device. The external card (secondary) drive is invisible to it. It’s like that with all players (not just the Clip+) and as far as I know, all car audio systems.
For me anyway, I found that getting a memory card and reader (or flash drive) that is as large as the player’s memory + the card (like 32GB) and using that in my car is better. You might want to consider this. Memory cards are dead cheap these days, and you can get micro SD card and very small reader than doesn’t stick out very far and just leave that in the car.
This is what I use:
I’ve got over 4100 tracks (.mp3 format @ 256 kbps bit-rate) on my 32GB card, and I find that more than sufficient.
Or you might want to use a miniscule flash drive like the SanDisk Cruzer Fit:
It doesn’t protrude our very far at all, and you could leave it plugged into the USB port all the time. People walking by your car when parked aren’t going to even see it.