Here's a re-post of the wiring diagram image now missing from earlier in the thread (I saved a copy right before it went missing due to the photobucket policy change):
After studying the diagram, it appears to use a slightly different labeling of the pins/wires than I used in my earlier post:
11-12-13-14-15-16-17-18-19-20
01-02-03-04-05-06-07-08-09-10
plug wires:
11-12-13-XX-15-XX-17-18-19-20
XX-XX-03-04-XX-XX-XX-XX-XX-XX
colors -
03 = white + black (WHT/BLK) = switch/circuit for 01-10? (LFO?)
04 = blue (BLU) = fogs (BFG?)
11 = light green (LT GRN) = high beams on (HU?)
12 = white + black (WHT/BLK) = switch/circuit for 11-20? (EL)
13 = pink (PNK) = light sensor? (?)
15 = green (?) = ? (?)
17 = violet (VIO) = high beams flash (HF?)
18 = red (RED) = tail lights (T)
19 = white (WHT) = auto (A)
20 = light blue (LT BLU) = head lights (H)
View attachment 258717
stalk pins:
11-12-13-14-15-16-17-18-19-20
XX-02-03-04-XX-XX-XX-08-XX-10
View attachment 258718
connections (P=plug, S=stalk, X=neither)
---------------------------------------
PS-PS-PS-XS-PS-XS-PS-PS-PS-PS
XX-XS-PS-PS-XX-XX-XX-XS-XX-XS
So, here's my latest theory based in part on the wiring diagram:
The tail light wire (RED) needs to be connected/switched on in order for the ECU to know to read the light sensor and toggle between DRLs and headlights. It makes some sense in that if the tail light wire is not engaged, the truck would have just headlights and no tail lights - not a proper nighttime setting. I developed this theory because the wire numbers in the diagram are difficult to read at the originally posted resolution and I thought there was some small chance the T/RED (T for TAIL) wire might be numbered something other than 18 (where it is in my harness plug). Comparing the T/RED number label to other pixel blended jpg numbers elsewhere in the diagram, I thought there was a small chance it might actually read 14, which is not only an open slot on the harness plug but would put the red wire next to the pink wire which appears to be related to the light sensor (confirmation bias?).
I don't know if this theory is wishful thinking or if I'm on to a solution. I doubt the theory because the diagram indicates it's for both "W/ AUTOMATIC LIGHT CONTROL" and "W/O AUTOMATIC LIGHT CONTROL" (which means it's not necessarily exclusively the Limited circuit diagram as indicated in the original post) and it doesn't: 1. differentiate between red wiring locations; 2. show a connection between the T/RED wire and the AUTO(3)/DRL(4) setting on the SWITCH ASSEMBLY. In any case, I see 3 paths forward: 1. As before, someone with a Limited or TRD Pro posts a picture (or description) of the wires running into their light stalk harness plug for comparison; 2. [MENTION=83083]rigtec[/MENTION] confirms the T/RED number on the original copy of the diagram; or 3. Someone (honestly and knowledgeably) indicates to me I won't destroy anything by pulling the T/RED wire from slot 18 and putting it in slot 14 (I suppose there's a circuit tester version of this experiment too). Thoughts?
EDIT: So the T/RED wire theory is wrong. The pdfs in the following post from another thread show clearer wire numbering:
