Dumb towing question (3rd gen 4runner)

OK, so my 3rd gen, like most of them, has the factory tow bar with the 2" receiver and the 4-pin plug for the trailer lighting.

Have hooked a couple of trailers to the truck and noticed the same thing both time: The brake lights, turn signals, and 4-way flashers will light up on the trailer, but not the taillights. I even went so far as to purchase a "tester" and it confirmed the same thing: No taillights.

Now I'm assuming that this means something is wrong WRT the trailer electrical connection, right? I did buy a new plug for the truck, but I haven't had time to wire it in yet. I guess my first step should be to trace the wires back to the source (looks like they spliced into the taillights on the left side, but I'm going to have to drop my spare to follow the wires.) Then, I can see if the wire for the taillights is broken. If not, I guess I can try swapping out the 4 pin receptacle that is mounted on the trailer hitch.

Anything else I should look at? I didn't know if maybe there was something different because most Japanese vehicles use a separate brake light and turn signal, while the trailer (like most American vehicles) uses the same bulb for the turn signal and the brake light.

I noticed this "issue" last year but didn't think much of it because the only trailer I had was a small utility trailer that I only intended to pull during the day anyway. Now that we have the teardrop camper, though, the ability to pull at night is a must.

Thanks in advance for any suggestions...

Mine didn't come with the factory 4pin if there even is one. I tried the cheap route and spliced with separate wires and ran into the same issue. I went ahead and bought the converter that plugs into the factory connector behind the rear drivers side panel, just around the vent. I think I picked it up from Etrailers.com. $30-40 bucks well spent.

Now I have running lights, turns, brake and hazard on my trailer, which has the typical combined bulb taillights.

Pop the plastic out in side the drivers rear corner, and you will find the "factory" converter box. My factory one was fried as well when I got the truck. I just spiced in a generic from the auto parts store which failed after one U-haul trailer rental, at that point I opened it up and soldered on my own diodes to isolate the signals and its been fine.

Mine fried when I was using it to tow my boat.

I wasn't home when it happened, so I did a quick road fix. I tested wires before the converter and after, Tail Lights in, but not coming out. So I clipped the wire from the converter and put in a jumper, as a temp fix... But I never have replaced the box, the fix works just fine.

I went with the etrailer plug and play kit as well, forgot who makes it. Not sure why the original ones fail so often.