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You only need to loosen the trim around the top of the panel. That should give you enought room to ge to the light. I assume you have a 1998 to 2001 Explorer with the light in the tailgate.
I had the problem on my 97. The plastic trim has to come out on the inside but even after you get to the light you can't just replace the bulb. The dealer told me that the whole unit has to be replaced. The whole unit consists of the entire trim assembly across the top of the hatch. Their replacement cost for the "bulb" was $300 including labor. The part was over $200. They changed the design of the light in 98 along with the shape of the hatch.
The bulb is actually neon and is a one-piece assembly. You need to drill out rivets to replace the assembly on a '95 - '97. Likely your problem is the ballast, located behind the plastic panel and doesn't require rivet removal. Last I checked the cost of the ballast was about $80.
I replaced my 3 rd brake light on my 96 XLT and that did not fix it. It is a neon bulb and I really do not think they can go bad (unless the gas leaks out) Open up the inside plastic cover and look for a black box. That is your ballast I would bet that is what has failed. Plus it is the much cheaper of the 2 to replace. and easier also. I get my parts from www.fordpartsnetwork.com they are in Florida and do a bang up job.