You'll need to use some problem solving and thinking, as well as crimp some wires and such for this step. Again, I'm not liable for any damage, short circuits, fire, bodily harm, etc...
Wiring is based off 2015 Mazda 6 Touring. Sport and/or Grand Touring may be different.
A pretty bad schematic...
This is the relay you want, pull it out.
Use an add-a-fuse in the in-cabin fusebox behind the driver's side kick panel (You may want to buy the low profile ones.)
- The third slot was unused from factory but gave a switched 12v so I used it.
Next step is to run the aux relay power supply wire through the firewall. (There are multiple ways of doing this, but I poked a hole in the rubber grommet and poked it through; if you do this BE EXTREMELY CAREFUL of NOT damaging the bundle of wires inside.) That thick blue wire is for my audio setup... you want to look at the thin blue wire. This was the hardest part of the install.
Engine bay, battery removed:
I got lazy and didn't want to disassemble the fusebox... but it's up to you if you want to make a clean install and tuck the wire in the factory harness guide. This require loosening/removing battery tray.
Here's the end result of what goes on in the fusebox. Make sure connections are secure and any bare metal is covered as best as possible. If you use the right amount/gauge of wire, you can tuck the relay into the hole and the fuse box will close with no issues.
An idea of what you'll see in cabin. I have additional accessories and crap so there's more wiring floating around.