Hi James,
I am guessing it has a 4849 power unit and either a Cableform or Sevcon foot pedal with an 8 pin oblong plug, if that's the case the common for the direction switch goes to the foot pedal (and any interlocks that are fitted) and you can do a continuity check from the coils for the forward & reverse. The other 2 wires are probably for a reverse bleeper as the Cableform controller only requires 3 wires. Its quite an old truck but I will look to see if I have a wiring diagram.
Regards Shane