I had a similar issue but my Nexus 7 2013 16GB model would not show up in explorer either. In the device manager the Nexus 7 had the yellow question mark triangle icon. I uninstalled it in the device manager, unplugged it, plugged it back in and it loaded the driver properly.
The following might help.
Determining whether the Nexus 7 uses the MTP or PTP connection is done by following these steps:
At the Home screen, touch the All Apps button.
Open the Settings app.
Touch the Menu icon button, and then choose the command USB Computer Connection.
The two options are MTP and PTP.
Ensure that thereís a check mark by the Media Device (MTP) option.
Touch the Home navigation button to return to the Home screen.
You should use the MTP connection because itís the most flexible. If you find the Nexus 7 not recognized by your Windows computer, rework these steps and choose the PTP option.
The difference between the MTP and PTP connections are in how your computer recognizes the Nexus 7. If youíve set the USB connection to MTP, the computer is fooled into thinking that the Nexus 7 is a portable media player. For the PTP connection, the PC believes the tablet to be a digital camera.