That red connector with 3 female bullet connector ports is on circuit 19 (LBLU/RED) and is for the dimmable illumination lights that are on when the headlight switch is pulled out at least one click and dimmed by rotating the headlight switch knob/shaft.
This circuit starts at the headlight switch's internal 15A circuit breaker and comes off the switch at the switch's 'I' terminal. It continues to the fuse box where it goes across the 2.5A 'INS ILL' fuse and continues to the red 3 port connector. From here is connects to a single female bullet connector for the radio illumination and then passes thru one of the multi pin connectors that go to the instrument cluster to feed the cluster illumination lights. Typically the red connector would only be used for RPO/DPO accessories in the interior which required dimmable illumination when the headlights were on.
That red 3 port connector is shown in this diagram (upper left corner just below and to the right of the 'A' in the circle):
Okay. Engine Stop. ACA out of Detent. Mode Control, both Auto. Descent Engine Command Override, Off. Engine Arm, Off. 413 is in.
-Buzz Aldrin, Apollo 11 LMP
Actual first words from the surface of the moon
Early Fox: 5.0L HO SEFI conversion, 5-speed, 8.8 w/3.27:1 TL, full dual exhaust
1968 coupe - Ordered 12.15.67 Delivered 2.1.68: 8T01T, 65C, 289 conversion, hanger queen