This is how I would approach the problem:
Start on a day when you are well rested and have enough time to devote to troubleshooting. Don't rush. Start at one end and work towards the other. I always start at the source and work towards the problem. In vehicle wiring, sometimes you have to omit the middle until the end. Beg, borrow or steal the right test equipment and know its limitations and become familiar with its operation.
1. Never assume. This includes assuming the fuses are labelled correctly. You really have to check all of your fuses and make sure you have found all of the fuse boxes. Sometimes additional electrical devices are added later and tied into a variety of sources, some appropriate, some are not. Some labelled, most are not.
2. Start with a multimeter or a 12v test light. Test your test device before you start. Don't assume that it is working. At the fuse box - ground one end of your mm or the 12v test light. If you look closely at the top of the fuse, you will see a small metal contact point on each side. With the fuses still intact, touch both recessed metal tabs on each fuse one at a time for voltage (light lights).
3. You should have voltage on each side. If you have voltage (light lights) on one tab and not the other, you have a bad fuse, no matter how good it looks (as long as you have good contact with your probe).
4. If you have NO voltage on both tabs, the following could be wrong:
a. Not making good contact on the fuse tab - covered in plastic perhaps?
b. Are the outlets energized only in the aux position of your starting switch and you are checking with key off?
Another common problem due to assumption.
c. There is a problem in the fuse box area that needs addressed.
d. You have a bad ground on your testing device if all the fuses have no voltage on both sides.
5. Once you have confirmed that you have voltage leaving the fuse box, proceed to the outlets.
6. Take a flashlight and visually inspect the outlets: Are the contacts (metal sleeve and center button) free of rust or oxidation? This is a common problem.
7. What are you using to check the outlets? Make sure it works in another outlet first. Sometimes the plug has an internal fuse in the plug tip that could be blown if your test device does not work. Is your test device of sufficient length to reach the center contact of your broken outlet?
8. The best way to check the outlet is to use your test light or mm. Be careful, since it is real easy to slip off the center contact and short out the outlet. There is a bright side - if you see sparks....you can locate the fuse more easily and your outlet was working.
9. If your test light does not light or mm indicates no voltage, first try a different ground for your test device and check the center pin again. If you have voltage then your outlets have lost their ground. Sometimes, a quick tab is used and it comes off of the sleeve. Time to tear into the dash!
10. If you still have no voltage, then you need to access the outlets from behind and look for wiring issues. Sometimes it is easier to run new wire than find the problem with the existing wire. Sometimes the 12V wire to the center pin comes off or overheats. Look for melted insulation, discoloration at the connection point at the outlet, and feel for stiff or brittle wire. If you have to do a splice, you have to make sure you cut out the section until you get back to clean and healthy wire. If you have an overheat condition at the connection point on the outlet, sometimes you have to replace the entire outlet due to excessive resistance at the connection point. Don't forget to use your nose!
11. Remember rule 1.
Hope this helps.