Thunderbolt ethernet en3 interface not appearing on MacBook Pro Retina

Question

2012 retina MBP, with a Thunderbolt ethernet adapter.  Fully up to date, including this week's Thunderbolt firmware update.  Plug the Thunderbolt ethernet adapter into any port -- the kernel sort of sees it, but nothing ever provides a network interface.
Infuriatingly, I got it to work once after weeks of failure, but have never been to reproduce that fleeting success.
I've seen and tried the comments in three seemingly-relevant threads:
https://discussions.apple.com/message/19550825
https://discussions.apple.com/message/21335473
https://discussions.apple.com/message/17909404
Like other reporters of similar problems, it's visible just fine in the system info:
Thunderbolt to Gigabit Ethernet Adapter:
  Vendor Name:    Apple Inc.
  Device Name:    Thunderbolt to Gigabit Ethernet Adapter
  Vendor ID:    0x1
  Device ID:    0x8003
  Device Revision:    0x1
  UID:    0x000100020014E9B0
  Apple Serial Number:    DTN22060026DWKHAR
  Route String:    3
  Firmware Version:    5.5
  Port:
  Status:    Device connected
  Link Status:    0x2
  Port Micro Firmware Version:    0.0.8
It's getting power, at least, because the switch link light comes on when a cable's plugged in.
And the kernel recognises it in either Thunderbolt port, at least to some degree:
IOThunderboltSwitch(0x0)::listenerCallback - Thunderbolt HPD packet for route = 0x0 port = 1 unplug = 0
[ PCI configuration begin ]
PCI configuration changed (bridge=58 device=5 cardbus=0)
[ PCI configuration end, bridges 14 devices 16 ]
But no driver ever gets loaded, and the en3 device never appears.
I've tried installing AppleBCM5701Ethernet.kext manually, which does nothing apparent.  Unloading and reloading produces:
AppleBCM5701Ethernet:     ffff     ffff getAdapterInfo - Vendor is unknown
AppleBCM5701Ethernet:        0        0 start - getAdapterInfo failed, giving up
It's unclear if any of that is relevant or related to this adapter.
I've borrowed another physical adapter and tried that; it gives the same result.
https://discussions.apple.com/message/21335473 suggests creating a new Network Location.  When I do that, the Thunderbolt Ethernet device (which does exist in my "Automatic" location) doesn't even appear in the new location.  Clicking the [+] does not list it as an option to add.
https://discussions.apple.com/message/17909404 suggests resetting the SMC, which I've done in theory -- although I can find no documented way to know that the SMC reset actually occurred.  Certainly there was no indication that I could see.  So I'm not convinced that it did anything at all.
I'm out of ideas.  This seems to me much more likely to be a software issue than hardware, at this point.  Almost like it just can't figure out which driver it needs to load.
I humbly throw myself upon your mercy and wisdom.

Answer 1

I had this problem, too, and an Apple Support call and Genius Bar visit later, I had the answer: if you're using Network Locations that preexist your Thunderbolt display, it doesn't seem to auto-add the possibility of "Display Ethernet" to your available network ports. To solve this, either:
Create a new Location in your Network system preferences. It will finally detect the "Display Ethernet" and use it. It'll slowly change from disconnected to No IP to green-is-for-go.
or
Fix an existing Location by clicking the + to add a port and choosing Display Ethernet. It'll slowly change from disconnected to No IP to green-is-for-go.