What Jethro says makes sense if the ground is just there for the indicator bulb in the turn signal unit.
A 4 (or 3) wire turn signal unit like your Guide needs separate bulbs in the rear for turn and brake lamps. It will not support using the same bulb for turn and brake lamps in the rear. Your American Autowire 510008 harness expects to use a turn signal switch that does use the same bulb for turn and brake lamps in the rear.
Any Guide 6004 wiring diagram I can find on line is a 7 wire unit.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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