Ford F series Owner's Manual page 211

Trailer lamps
Make sure your trailer lamps conform to Federal
and local regulations.
Your vehicle has been equipped for use with a
4-wire trailer tow electrical system (running
lamps, right hand stop/turn, left hand
stop/turn, and ground).
Class I and Class II Trailer Tow electrical wiring
provides two circuits (right hand and left hand)
to operate trailer stop/turn tail lamps. Each
stop/turn circuit will operate one combination
stop/turn light bulb (# 1157, 3157, 2357, or 3357)
on the trailer. Never add more than one trailer
light bulb to one circuit.
A wire harness (Ford part number
F5TB-13A576-A) has been provided that connects
to the frame wire harness at the rear of the
vehicle and is compatible with most 4-wire
trailer tow electrical systems.
Always check trailer and vehicle functions (i.e.,
stop/turn signal, and running lights) before
operating your vehicle and trailer.
If your vehicle fails to function properly, do not
operate your vehicle and trailer until corrections
are made.
Always replace a fuse with one that has
the specified amperage rating. Using a
fuse with a higher amperage rating can
cause severe wire damage and could start
a fire.
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