*01/07/2023 - Edit: added additional information to wire components in engine to the BJB (battery junction box) on post #28
You have the power! Well okay it's really the BMC but you helped. (If you don't get the reference then I feel sorry for your childhood)
Okay there seems to be a lot of confusion surrounding this topic. New and old members are constantly asking were to find either switched power (ignition) or constant power (battery hot) for one reason or another. Regardless the reason there is a correct and incorrect way of achieving this.
Most members do not feel comfortable splicing into physical wiring and truthfully I suggest avoiding it unless necessary (why add potential issues if it can be avoided). Scotchloks & T-taps are for scrubs (yes I'm talking to you!)
Fuse Taps are a good way to add additional components while leaving the factory wiring another. However there is a correct way to install these to prevent causing damage. If you're read this I'm going to assume that you have some method of reading voltage. It doesn't need to be an expensive DMM (digital multimeter) but something that shows you are working in the right direction.
So let's start with this example:
You want to install a dash camera because the general public is comprised of savages.
For this camera you need to have a constant source of 12 VDC power (battery hot) and a ignition source (a signal that indicates the vehicle is in use). Well unfortunately unless to are looking to run additional wires to the BJB (battery junction box) next to the battery a true ignition source isn't truly available at the BCM (body control module) but using a signal that is connected to the interior lighting circuit is the next best option.
The majority of Focus ST's in the USDM have an unused Fuse slot on the BCM that is connected to a constant battery hot so that will be you connecting point.
It is up to you to verify that the BCM Fuse slot has Fuse blade supports. Not all empty slots have them and if you attempt to press a fuse tap against the BCM's circuit board damage could occur
So let get things hooked up.
BCM (F61) = front and rear power points (hot whenever the vehicle is operating and off when exiting vehicle afte the engine is turned off)
BCM (F88) = Unused per ford. This is open to add additional components not exceeding 17 amps (85% of 20 amp maximum) *circuits exceeding 20 amps should be run directly to an open point on the HVJB (high voltage junction box) but that's a different post for a different time.
Do not add fuse to lower slot in the fuse tap for F88. It will add power to components not use and could cause issues. Better safe than sorry.
Attention!
It's important to understand how a fuse tap works. Installing them incorrectly can lead to damage or negatively effect other components on the circuit you’re tying into.
Correct orientation: The side the does not have the wire is to be placed on the source voltage (switched or constant). This allows the factory circuit to be left alone (Fuse 1) and allows for an additional component to be added on the circuit (Fuse 2)via the attached wire.
It is up to you to verify which side is source voltage and which is the load side (component) by removing the fuse and checking each side of the fuse holder. You have been warned for the last time.
If the fuse tap is incorrectly installed all the load is carried by (Fuse 1) as it becomes the source voltage for everything.
Wanna see what 9.5 amps on a 10 amp fuse looks like? Sure it worked for awhile but after a month this persons vehicle still smells like burning plastic. They were lucky as I've seen several vehicle fires in my career. One being fatal. Take the time to do it correctly!
So the above should get you what you need. All that's left is a good ground point which can be easily located at a factory location behind the panel on the passengers side footwell next to the BCM.
Hopefully this helps and if you have any questions I'm always here to help whenever I can.
Stay Safe
J