So this is either a puzzle that starts with a Wherigo, or a Wherigo that ends with a puzzle. In either case, you will need to complete the reverse Wherigo first. At that location, will not be the final, but a field puzzle. Once you have divined a solution to the field puzzle, you will be able to go to the final location and sign the log. In my mind, the field puzzle and final location are a walkable distance, but of course, you can choose to drive the short distance if you choose. Of course, if you can't solve the field puzzle on the first trip, leave the puzzle but take a photo to work on and come back later.
For the reverse wherigo, I'll be generous and give you the three codes that you need: 703349, 752709, and 955397.
The Reverse Wherigo, aka The Reverse Cache - beta, can be downloaded here.
Refer to this FAQ for more info about the Reverse Wherigo.
You can verify the solution of the field puzzle here:
You can validate your puzzle solution with certitude.