Advantages of Wiretapping
Tip: You can track Collectibles and consumables by pressing X on the collectible to mark it in your map. This will give a distance marker which will guide you to the collectible's exact location
TL49-Fuses
Three of these fuses are needed to wiretap a junction. There are a lot of these scattered in various locations in the city. They also appear as small green dots in the map if they're within the wiretapped area. Take note that there's a lot of spare fuses compared to what you need to wiretap all junctions in the entire city so there's really no rush in collecting all of them immediately.
Junction Boxes
Once you unlock the story missions dedicated for a district, all Junction Boxes will be revealed in the map. Some junction boxes are inside police or hostile territory so it might pose a challenge to wiretap them without some kind of preparation or Combat.
Tip: Press L3 to toggle the normal map overlay or to display the areas of the district that is successfully wiretapped.
How to Wiretap
Basically you just need to break the junction box's cover open, with the similar mechanic as to opening locked doors, then interact with the opened junction to place the wiretap. That's how easy it is. Now getting to the some junction boxes will provide ample challenge especially when they're guarded by the police or hostiles.