Can I rig up my own “converter” with relays and such?

91 Nissan truck that had a 4pin flat trailer plug. They have that weird 5 wire system that generally needs a “converter” to adapt it to the normal 4pin functions. (Something about it has a separate bulb for turn signals, instead of being combined with the brake/tail light bulbs.) It had been hacked in the past to work but when I got it, it won’t feed any power to the harness plug. I’ve finally found the vehicle function wires and traced them to verify they match the tail light functions…
Since it was “hacked” I don’t believe a commercial converter would be useful unless I was to do more research and try to put it all back to stock condition, but even that would be a problem, as two or three wires had to be accessed further toward the front end, due to shorts/corrosion and such.

Splicing the trailer harness into the rear bulb socket assemblies isn’t that simple, I believe I’ve seen in videos, because of “signal flowback“ or similar that messes things up when the turn signals are operated at same time with other functions…

 

There are powered "combiners" that use battery power to power the trailer lights. You would have to tie into your brake and turn signals, but they just serve as on/off signals to the unit. I'll see if I can get a part number in the morning of the one here at work.

Relay logic will be a bit of a headache. Maybe with some separate logic gates it could be worked out also.

 

After the sun came out I went out and looked at the trailer. Unfortunately the "box" must be in the boss's office. Long story short that particular trailer he fitted to work on either a semi truck (separate brake / turn lights like your Nissan) or his pick up truck (normal single light for both functions). It was a matter of getting a "combiner" to combine the brake and turn signal functions into one light. Many parts stores sell them, but I think this one wasn't designed to just plug into the factory harness. It was more for someone to wire up themselves. Maybe he just found a good sales person, but I remember hearing the non powered ones usually don't hold up too well. The one he found was designed to use battery power to power everything... kind of overkill on a semi since trailer lights are normally straight from the battery and through relays anyways, but it didn't seem overly expensive. You'll probably have just as much in relays and other parts to build your own.

 

Just to kill a few more minutes while waiting on a few things I did a little looking around online. I didn't check any reviews, but this one is pretty much the only one I could find that uses separate power. https://www.etrailer.com/Trailer-Wiring/CURT/C56496.html?feed=npn&utm_source=google&utm_medium=cpc&utm_campaign=Google | Shop - Trailer Wiring&adgroupid=128484934262&campaignid=15586143335&creative=569877447301&device=c&devicemodel=&feeditemid=&keyword=&loc_interest_ms=&loc_physical_ms=9015028&matchtype=&network=g&placement=&position=&gclid=EAIaIQobChMIx6ua8YfQ9wIV1gorCh1Uig0nEAQYASABEgLKyPD_BwE

Edit: There is also this route... http://hhrvresource.com/sites/default/files/Images/fourrelay.jpg I changed my mind.... It draws on the original circuits instead of remote power

 

Just to kill a few more minutes while waiting on a few things I did a little looking around online. I didn't check any reviews, but this one is pretty much the only one I could find that uses separate power. https://www.etrailer.com/Trailer-Wiring/CURT/C56496.html?feed=npn&utm_source=google&utm_medium=cpc&utm_campaign=Google | Shop - Trailer Wiring&adgroupid=128484934262&campaignid=15586143335&creative=569877447301&device=c&devicemodel=&feeditemid=&keyword=&loc_interest_ms=&loc_physical_ms=9015028&matchtype=&network=g&placement=&position=&gclid=EAIaIQobChMIx6ua8YfQ9wIV1gorCh1Uig0nEAQYASABEgLKyPD_BwE

Edit: There is also this route... http://hhrvresource.com/sites/default/files/Images/fourrelay.jpg I changed my mind.... It draws on the original circuits instead of remote power

That ETrailer kit is probably my best option, even though it “goes against my grain (hard-headedness) for not being able to build something from scratch that would be cheaper (again, hard-headedness or lack of reality lol)”…
There was one YouTube video where a fellow built a “unit” that tucked inside the truck bed. It was a relay bank (I think 4 relays total) and some diodes added in to stop the flow back or quieten the relay clicking…it was a little deep for me at that moment but with a couple more views I could probably grasp the process… but like you say, could end up with just as much in parts…but look at the fun! LOL
Thanks much for the ETrailer part info. Likely the way I’ll go.

 

It's always more fun to build your own, but sometimes it's easier and simpler to find an off the shelf part. Good luck