Well since the trailer connector (4th brake lignt) doesn't work but the 3rd does, it's likely a wire issue somewhere between the cab and lamps. Could be either the switched +12 or ground wire(s). Both lamps may have failed, but not likely. As far as removing the light assemblies... in addition to the screws there are "ball-snap" type fasteners. Gently pull or pry the lens assembly back and to the right or left depending on which side you are working on respectively.
Y2K, Oxford White, XLT, S-Cab, 4.2l V6, Auto, Sliding rear window, Captains chairs, Med. Graphite int., Premium Audio; 5-spoke cast Aluminum wheels.
Oxford White grille surround w/"Storm Grey" Honeycomb insert.
Mud Flaps, Bed Liner.
Tinted Windows; 2x55W Auxillary Back-Up Flood Lamps, 2x55W "puddle" lamps, 2x55W Projector Beam daytime running lamps, 2x27W Under seat rear floor lamps, 2x27W Kick panel floor lamps; UNGO Keyless Entry; WeatherTech window deflectors.
Chrome mirror covers painted "Satin Black", Embossed FORD logo on hubcaps painted Black.