I actually ended up using a different one for accessory power because the fuses located just above 66 (56, 57, 58) interfered with the fuse tap seating all the way into the slot for 66. I just poked around with a multimeter to test which ones were constant/ignition/accessory power and which ones were constant, then looked up what each one was for to make sure adding an amp of load wouldn't cause a major problem.
If you have a multimeter with nice sharp probes, you certainly don't have to take the fuse out to test them. On the top of each fuse (where it says 10, 15, 20, etc.), there are two tiny indents where you can touch your multimeter to the fuse and see whether it's powered (just ground your black lead to ground somewhere on the truck). Makes it very easy to test fuses quickly. I stuck my black lead up into he middle of the spring that's above the fusebox so I didn't have to hold it in place, then used the red lead to probe each fuse to determine if it was constant, accessory, or ignition power.
B