Lazy Daze Owners' Group

Hi Jane; Older systems are often not well understood.  Insisting on a bad circuit board; I don't think my '99 has any circuit boards involved with trailer lights.  Europe, and now many cars have three wires. Separate brake, running lights and turn signals, usually amber, for each side.  The American standard was TWO wires.  The running (sometimes called parking) lights.  And the Brake lights, which do double duty as turn signals, by turning off the brake light only on the side signaling a turn, if the brakes are on, or by turning on the brake light on the signaling side if the brakes are off.  That is what Ford, and most american cars do.
   The four wires you have are Ground (White insulation);  running lights ( Brown insulation); right turn signal/brake (Green insulation); and Left turn signal (Yellow insulation).   Lazy Daze used a light brown wire for tail lights, license, front corner lights, 5 amber markers, and 5 red markers.
    There is a standard 7 wire implementation plug and socket, that Ford and others have adopted.  The definition of which wire goes where is still open for some interpretation.  https://www.etrailer.com/question-363.html  has a write up for this. This type of 7 pin connector can be purchased many places (WalMart) for less than $20. and has worked well for me.
     Ford used, until about 2003, wires for the tail lights that were too small to carry the 8 Amps necessary for proper brightness of the brake lights. 2 Amps per tail light times 4 brake lights left not enough voltage. (about 7.4 v.) LED brake lights don't draw as much current when on.  3/4 Amps per light. 3 Amps total for four lights, instead of 8 A.  Often a buffer relay (real mechanical relays), or more solid state relays can provide power to a towed vehicle lighting system.  In the old days (so called steering) diodes were installed to separate the towing (motorhome) from the towed (dinghy).  One problem with the Ford was with the cruise control not working. The decreased current draw with LED lights was interpreted as a burned out light. Some people substituted load resistors to keep the current drain the same, but that isn't the right cure.  A new electronic flasher relay fixes that problem, and provides consistent timing.  The Grote  Amazon.com: Grote 44892 3 Pin Flasher (North American (JSO) Pinout) : Automotive   is what I have used.  
    A link to wiring I put in in 2017 for a 2001 MB:    Brighter tail lights recs?      RonB