Everything CWF says and:
Everything cwf says plus:
1) Check your tester to make sure it works. There’s nothing like poking things, including your fingers, before you figure out your tester is bad. Ask me how I know this.
2) Check continuity of your ground wiring. If your trailer is not grounded to the tow vehicle thru the ground wire, you can have bizarre electrical behavior. Unhitch the trailer to eliminate grounding thru the hitch, safety chains, or weight distribution bars.
When you check for the turn signal at the truck’s 7 way connector, use the connector ground pin for your ground. You need to verify that the truck 7 wire connector ground completes the circuit.
When checking the umbilical cord, do the same—use the ground pin in the umbilical cord connector for your ground. You need to verify that the umbilical cord ground completes the circuit.
Check the 7 way connector connections inside the trailer. There is a plastic cover in the front wall of the trailer, which, when removed, reveals the back side of the trailer connector the umbilical cord plugs into. Again verify circuit completion thru the ground post.
The trailer connector ground post is wired to one or more studs or bolts welded on the trailer frame. If the ground connection is intact, the trailer frame and inner and outer shells, are grounded, so your tester should flash when placed between the turn signal post and either the inner shell or frame.
One final comment. Exposure to the elements may have tarnished the pins and sockets of your umbilical cord , truck connector, and trailer connector. You can buy specialized cleaner tools to clean the tarnish. The pins of some connectors, like the old Bargmann, are split and can become compressed so they no longer make good electrical contact. Gently spreading the pins open with a thin bladed knife can restore proper contact. Be gentle—you can spread them too far or even break them.
"To err is human, to air is devine."