I have an older pioneer H/U with usb. I can get the model tomorrow as I'm in for the night.
Now I might have mistaken your problem. I thought you were having an issue with the radio recognizing your tablet. It does but you cannot get the radio to play your music files on your tablet. Is this correct?
My radio does recognize my phone but I don't use it to play music. I simply use a usb key. I should try it tomorrow to see what happens.
Now you mention something about bluetooth? Was that what you were originally trying to do?
Does this have any info that might help you -
Note 3: play music through bluetooth to car stereo? - Android Forums at AndroidCentral.com
Read this -
Galaxy S4 and playing music via usb in my car - Android Forums at AndroidCentral.com
Read this -
usb connection mode - Can I get my car's head-unit to play audio, via USB, from my Galaxy S3? - Android Enthusiasts Stack Exchange
Also found this on another site with regards to a person trying to get the radio to work with his android device via usb - "try turning on USB Debugging, it's under Developer options in Settings"
Do some research about your tablets android version and hooking up to a car radio via USB. Some versions did not support it. Something to do with mass storage or something along those lines.
You can't be the only one who has tried with your h/u and an android device, specifically with regards to USB. Based on my quick searches, alot of people have this issue.
Post up what android OS version your tablet has and I'll do more digging.