Stage 13 hints
The last stage, and this one uses a feature of the cache page.
The clues are:
Look Up - you are going to look up some things / use an indexing system
Time to reflect on what you have achieved so far - look back at all the caches in the series and look at the end of things that appear on each cache page. Somewhere there should be something matching the Westing puzzle.
Use your head very figuratively (unless really lazy) for the Northing. Just look at the first letters - HVF URL. That should point you at something.