GM used to be pretty good about providing switched and unswitched terminals in their fuse boxes, and they were labeled. If you have those, that's the best choice. The taps you're referring to do work but they can spread the terminal in the fuse box because you're, in effect, making the blade of the fuse twice as fat. That wont be a problem until you want to remove that tap, then you may have an intermittent connection on that fuse.
If you do use the tap, place it on the hot terminal. That way the added current for your new device wont go through the existing fuse and potentially overload it. To find the hot terminal, just pull the fuse out, then use a voltmeter or test light to see which terminal has 12 volts on it. That's the one to use.
Also be aware that some devices, like power inverters, can generate signals that interfere with the computers on the vehicle. If you start having trouble with the dash gauges or other things not working properly, try tapping into a different fuse.
Monday, October 1st, 2018 AT 11:14 AM