Wiring the the signal lights, stop lights, trailer brakes, flasher etc. on our 1926 Model T Pickup turned out to be a real pain. There were so many wires and connections that the only way to tame them was to run them through a 7 wire junction box.
Signal Switch – I ran a separate ground wire to the switch because there was no natural ground through the steering column.
Flasher – I used an LED only flasher with its own ground wire. The 12V input came from the firewall terminal block through the 7 wire connection box.
12V + I took a lead from the firewall terminal block through a 15A fuse to the junction box and attached the converter, beeper, and flasher to the box.
Brake Controller – the 12V input wire (black) and the output wire to the trailer brakes (blue) are both 12 gauge. I ran the trailer brake wire through a 20A breaker into a 14 gauge, 6 wire cable. I used the bundled cable for neatness. The 14 gauge wire may be undersized for the brakes but it is overkill for the LED signals. The factory wire leads from the electric trailer brakes are 14 gauge.
Trailer Light Converter – I chose to use 1157 lights in original tail light housings rather than separate brake and tail lights. That choice necessitated the use of a converter so the stop and turn signals could work on one element of the bulb.In any event the converter was necessary for the lights on the trailer.
Once I got everything wired strange things happened. When I put on the brakes all four signal lights came on. One or two individual LEDs in a couple of light fixtures stayed on all the time. Signal either way and all four signal lights flashed.
One of the problems was the LEDs I used for rear signal and stop lights. They are not true two element lights. A resistance circuit is used to differentiate between the bright stop and signal lights and the dimmer tail lights. For some reason this allows voltage to leak back in the system connecting all the lights together. I fixed this problem by adding diodes to the tail light inputs. I probably only needed one diode on the problem wire but I put a diode on each of the tail/signal light leads. The diodes were encased in heat shrink tubing to protect them from the elements.
When tail lights don’t flash properly on a trailer the best place to start looking for the problem is the ground. Based on this bit of experience I grounded everything I could. Still now satisfaction. Replacing the converter didn’t help. Still more trouble shooting and finally I tried a third converter. Problem solved.
The first two non-powered converters I purchased were faulty. The third one was a more expensive powered converter and it worked fine.
In addition to the other hardware I wired in a beeper that Joe Fellin gave me a few years ago. I tied it into the pilot light circuit. It was so loud that I put a piece of tape over the speaker outlet.
I also ran a separate 12 gauge wire on a 40 amp breaker to the trailer 12V system.