I started with replacing the 4-way to RV plug adapter; no change. (That goes back to the store.) Then I took off the RV adapter that was wired on to the trailer to look at it, and saw that only four wires (matching the colors on my TV plug) were wired into it. So I unwired it and followed those wires back to where they were spliced into the trailer wiring about a foot back. Voila! The yellow wire (left turn) was not connected to anything. I picked the unconnected blue wire on the trailer as my best choice, twisted it on, and I had a left turn signal. Wired it properly, then retaped everything and rechecked; all the lights
now work. Since that kind of left me with a mess of wires and sticky tape, I took a piece of PVC pipe, stuffed them all in, and taped it up and taped it to the trailer (will clamp it later.) Checked lights
again, they still work.
If I ever decide to go to a battery
(which won't happen unless I get a 3-way fridge
to replace the icebox) I'll have someone else wire it all in!