Find the source of power at a word wall location

Walkthrough
If the Dragonborn uses a shout within the boundaries of an inhabited area, he will subsequently receive a letter by courier urging him to investigate a source of power at a given location. This location could be any of the word wall locations.

Also, depending how many times a shout is used within city walls, the guards may well ask the Dragonborn to stop doing that.

Possible locations
Finde die Quelle der Macht an einer Wortwand-Lokation