Paperwork sorted
Just to finish this thread. I went back into Mexico and what do you know, there are signs to the aduana office from the border which is where you also need to cancel your temporary import certificate. A few signs before you get in the long queue to get out of Mexico wouldn't have gone amiss.
So I guess the easiest way to to find the aduana office in Tijuana (if you don't actually know where it is) is to go through to the USA turn around and come back into Mexico and follow the signs to the office. It really was very simple coming in from the other side. And if you lane split all the way to the front of the queue, dodging all of the street vendors, it only takes a little extra time.
They also told me (it turned out that I was a day over my 30 day visa) that they won't charge you unless you are more than a month or so over. They were very friendly.
Also, we did do some research before posting the original query about the US import docs and didn't find anything conclusive.
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