On my '05 a 20A Fuse #33 powers the running lights / tail lights. Pull the fuse and see if it is blown (hard to see from the top), while out test to see if 12 VDC is present at one of the terminals where the fuse plugs in (power supplied by 50A fuse #15, controlled by "Park Lamp" relay) when the parking lights are on.
If no power, check the 50A fuse. If fuse is good, issue is with the "park lamp" relay #73 (see the location map for the relay inside the "Fuse and Relay Center" cover that you removed to get to the fuses. Debug the relay or replace to get power to Fuse #33, problem solved. If still no power to Fuse #33, then issue is with the ground to the Front control module. Test and confirm, then it is either the wiring /connections to the FCM, if you are lucky, or the FCM itself.
If fuse 33 was blown, use multimeter (volt - ohm - amp) and check if there is continuity at the trailer plug between ground (white wire, terminal #1 on either 4 pin or 7 pin connector) and the running lights (brown wire, terminal #2 on 4 pin and terminal #3 on 7 pin). If continuity, find the short that blew the fuse and fix it first. One of the trailers probably has a short, once it blew the fuse running lights on all the trailers won't work. If no continuity on any of the trailers wiring is probably ok, but driving down the road it could move and short again (time for a package of 5 of those 20A fuses).
If no short and a good fuse in #33, see if there is power to terminal #3 of the 7 pin socket on the truck. If no power, check to see if the 10 pin connector is completely plugged into the back of the 7 pin socket on the bumper. Black wires at pins 8 and 9 of the 10 pin connector are grounds, brown wire at pin 5 is the power for the running lights.
Further up the rear body wiring harness from the 10 pin connector is where a 4 pin trailer socket is spliced in and stored up behind the bumper and trailer hitch platform. Test for voltage between terminal #1 and #2 of the four pin socket to determine if there is power for the running lights. If there is, then just use the 4 pin socket and plug the trailer directly into it using the 4 pin plug on the trailer until you fix the 7 pin issue (assuming you are using 4 pin to 7 pin adapter in your 7 pin socket on the bumper).
If there is no power at either the 4 pin socket on the truck or the wire going into the 10 pin connector, and you have power on the other side of fuse 33, it is probably worth the appointment to find where in the wiring harness the wire is broken and how to fix it.
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