A bit late here I think, but I fitted a 7" Nexus 7 (2012 model) in mine!
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Built in GPS and WiFi is great! Does every thing I need, and to cheat, I used the (no longer) empty space behind the tablet to hide this:
NEW Car Audio Stereo In Dash Fm Radio W/Mp3 Player/USB/SD/Input AUX Receiver UK | eBay (A £15 stereo that is Radio and AUX in (NO CD))
Then I used the headphone out of the Nexus into the Aux In. Cheaper than using a DAC or anything from the Nexus.
Lots of swearing, cursing and huffing but I'd still happily help someone else if you wanna give it a go
If using a Nexus 2012 I can provide all files used including TimUr's USB Host Rom (means the tablet goes into standby when the power from ignition if switched off etc when using an OTG-Y cable) and all files to make it look how mine does