Bwana50, I'll try. The colors I give are for an '89 harness.
First the control circuit. I spliced into the R/W wire that supplies power to the hazard flasher from fuse #1. From there I was able to run power to one side of my SPST switch. The other side of the SPST switch goes to one side of the coil on the relay. The other side of the relay coil hooks to ground. I also hooked a EMF diode between the two wires that I ran to the relay coil. This prevents arcing and damage to the switch due to the collapse of the coil field when the switch is turned off. (A more detailed explanation is a post of it's own) The diode should be installed with the negative end (end with the stripe around it) attached to the wire that supplies power to the relay coil. The other end attaches to the coil ground wire.
The 3PDT relay has 3 power input terminals and 6 possible power output terminals. (this is not including the coil terminals) Three of the outputs are "hot" when the relay is energized (coil is powered). The other three are "hot" when the relay is relaxed. (does that make sense?) Attach the LB wire from the Mustang hazard switch to one of the power inputs. Put the O/Y wire from the Mustang switch on the output terminal that will connect to the LB wire when the relay is relaxed. Next, attach the W/R wire from the donor switch to both of the remaining power input terminals. Attach the O/LB and W/LB wires to one of the output terminals that will receive power from the W/R wire when the relay is energized. Attach the LG/O and the LG/W wires to the other output terminal that will receive power from the W/R wire when the relay is energized.
A 3PDT relay has a total 11 terminals on it. In this application you should have 3 unused terminals.
Any questions???? Feel free to ask. It is hard to describe an electrical circuit in text. If this is completely unclear, perhaps I can get a friend to scan my schematic onto the forum.