1995 Ford F350 having issues with marker lights and left turn when hooked to tow a trailer. Everything works on the truck when NOT pluged into the trailer, but when connection is made the only lights that work are the right turn, brake and trailer brake lights. The truck has a factory tow hitch and ALL lights worked just fine 2 days ago.

Check the bulbs and grounds at the trailers marker lights.

Troubleshooting No Running Lights on Trailer but Turn Signals and Brake...
http://www.mastercraft.com/teamtalk/archive/index.php/t-17703.html
Thread Running lights on trailer don work but turn signals and brakes do

have you checked the tow tail light fuse or the tow tail light relay?

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

make sure to check the wiring on the trailer and to make sure that there is no damage or loose wireson the hook ups themselves. the wres could have come loose on your truck where they hook into the hook up for the trailer wiring. if any wire has come loose or the ground wire for the lights have come off, they will not work.

If you have your owners manual it will be in there.

your problem is discussed on this link,, scroll down to the 2 and 3 wire hook up systems, and I'm sure the info you need is there , if you have any questions just reply to this feed , http://www.accessconnect.com/trailer_wiring_diagram.htm

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 the fuses as there is separate fuses for left, right and tail... they're located in the engine compartment fuse box.

Battery junction box (BJB) fuse(s):

• 5 (30A) (trailer electric brake controller)
• 34 (25A) (trailer tow turn relays)
• 42 (30A) (trailer tow parking lamps)
• 44 (30A) (trailer tow battery charge relay switch side)
• 63 (15A) (trailer tow reversing lamps

Although it IS VERY Straing that Trailer Lights Work I would track down the Harness And Check All Plugs if they are all CLEAN then I would Suspect the HEADLIGHT SWITCH

I take it the blinkers and brake lights work ok when not hooked up to the trailer? If yes, then the wiring to the tow hitch is not hooked up correctly. How many wires to trailer pig tail connection? U can have up to 7 depending on what you tow. Check to make sure these wires are hooked up from the truck to tow plug correctly: 7 wired---
brown-Rt turn and brake light, yellow-backup, white-ground, blue-electric brake, green-tail running lights, black-battery charger, red-left turn signal and brake light. Let me know if u have something other than 7 wire, I'll get the wiring color and source info for u.