When I HOOK UP MY TRAILER LIGHTS TO ANOTHER VEHICLE THEY WORK FINE, but when I hook them up to my 98 expedition my trailer park lights do not work but the brake lights and blinkers work fine. I checked all the fuses under the dash and they are ok. I do not have the owners manual to the vehicle. Checked the connectors that go to the trailer plug and they appear to be ok . Any ideas on what might be my problem?

Congrats, you have found Ford's super secret fuse that drove everyone mad for years!

Make sure the trailer is grounded

This is most likely going to be an issue with the wiring to the trailer conector on the vehicle or the trailer conector on the vehicle

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IF you have this trailer with this vehicle before the problem will probably in the wiring in the plug. Loose broken or corroded away. Check condition of the light sockets and earth points and wiring back to the plug

Have you checked fuses? the most common problem is a bad fuse, make sure to check the fuses located under the hood ,check them all, there are light modules and other components that run power to tail lights. also if you use the tow hitch and light hook up and the fuse is bad then also check and see if fuse pops with trailer connected ,if it does then check wiring on trailer lights

Check fuse 28 in the fuse block under the left side of the dash. There is also a relay for the trailer park lamps in that same fuse block. The fuse and relay are the trailer parking lamps and run independantly of the vehicles parking lights. You can also try backprobing the other trailer plug (you should have one 7 pin and one 4 pin) to see if there is power at the park light pin. If there is power at the other trailer plug than the fuse and relay are bad and you may have a bad/corroded plug or bad wring to the plug that doesn't work.

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your vehicle has a "integrated power module". its a fancy name for a smart fuse box. What likely happened is there is a problem with the trailer wiring. The power module detected the fault and shut down the running lights. The power module will need to be reset at the dealer.

Fuse 15 is a 50 amp fuse for the park lamp relay
Fuse 32 and 34 are 10 amp fuses for the right and left park lights
Fuse 33 is a 10 amp fuse for the trailer park lights.

After it is reset I would not plug in the trailer until the fault is located and repaired.

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Did you check the fuse for the trailer tow adapter parking lamps ? This is a separate fuse from the truck's parking lamps.
Fuse #10 is the Trailer Tow parking lamps ( via the trailer tow parking lamp relay ) and this is the same fuse for the trailer tow reversing lamps ( via the trailer tow reversing lamp relay ).

If you did not get an owners manual with your truck ( purchased used ), you can download one from Ford :
https://www.fleet.ford.com/maintenance/owners_manuals/default.asp

Diagrams for the Trailer Tow adapter :

check you bulb and there sockets its possible that you burned out one of the two

Did you guys ever solve the issue with your trailer wiring? I have a 2000 Expedition with only one brake light on three different trailers no other lights work. They were working fine and the work fine on other vehicles. I know it could be anything but I'm not sure if it's a known issue that the plug by the rear wheel back to the adapter gets bad or if there is a typical issue that lies elsewhere. I'm just trying to find where to start????