Check to make sure the connections in the glass fuse holder are actually touching the fuse end.
With your meter, just set it to read volts and put one lead on the battery negative terminal. Then take the other lead and backprobe each connection of your wiring. Start at the connection closest to the power supply. In your case, start at the fuse tap. Make sure you're getting voltage. It should be about 12.5-13V. If you have voltage, then move to the next connection, and so on and so forth. When you get to the connection where there is no voltage, then you know your fault is somewhere between that connection and the previous one that tested good.
Also, make sure the circuit you're tapping into is actually hot when you want it to be, and actually hot when you're testing it. For example, if you chose a circuit that is only ON when the ignition is ON, then make sure you're not testing it with the ignition OFF. It sounds simple on paper but we all have brain farts and I've made this mistake countless times.
Edit to add: Before doing any of that, I would take a jumper wire and hook up your camera directly to the battery. If it does not power up, then it is a faulty camera. If it does power up, then you know the fault is somewhere in the wiring. Sorry, I kinda went backwards in the order you should do things.
