DRL mirror switchbacks killed my truck

I think we have eliminated the possibility of the problem being in the MBECU or the Junction Block or the Combination Selector Switch.

1) The MBECU is receiving the signal from the Selector Switch and activating the Relay. The relay activates both your Dash Lights and your Tail Lights.
2) The selector switch is working because your Dash Lights come on
3) The Relay works because your Dash Lights come on.

This leads me to suspect the wire is broken at the T-Tap.

 

I'd suggest you save some $$$ for a dealership diagnostic. Let them put in writing what they recommend, then take it from there...

 

Are the 1F and 1A outputs on the same connector body?. Could you please post pic's of that / those connectors? Just curious if it's possible to access and test those pins with a DMM while connected to the Junction Box?

 

OP, one more test to verify if the MBECU is working and only the board trace to the Tail & Panel are affected.
In the engine room fuse box, locate the Towing Tail Fuse and remove it. Locate and remove the Towing Tail Relay. Identify the #1 pin slot, using the Multimeter, check continuity when steering stalk is Parking selected.



Or get a towing tail light checker to see if you get TAIL illuminated in the tow connector at the rear bumper....

 

I agree with this assessment.

 

Looks like caribe has been holding back on us? Still could be a relay ???????

 

@caribe makaira yes there is power in the #1 slot.
@PhenixFord what do you mean the 1f and 1a connector body? And I will remove the connectors from the orange wire shortly, inspect the wire, worst case I'll have to butt splice a little pigtail on, but I'll let y'all know if removing that connector does anything

 

Dash lights dim when Parking position is selected...verified OP?

 

There should NOT be power in slot one of the Towing Tail relay...MBECU provides ground to the relay when Parking is selected...

 

Sorry Brother scratch this (1F &1A). I relying on carib's expertise and wiring diagrams. He has posted new material since this discussion.

It's possible that the new relay is bad. Or the wire is broken at the t-tap. Just removing the tap will do nothing to find the problem. It will be necessary to remove the insulation from the wire where the t-tap was applied to verify that the wire isn't broken.

 

One wire in a parallel circuit does NOT cause all other lights to fail...if the tap is not connected to ground...

 

It will if [EB1] is broken.

 

@JordanJake25

Just for fun - Pull the TOWING TAIL RELAY. And put it in the HORN RELAY position. Then blow the horn.

 

... ok... Fuck.
@caribe makaira I will get out there and recheck that relay slot 1...
@PhenixFord I'll remove the t-taps and check the wires... PS there's one on each side, not sure if that was obvious or not... Annnnnd to see if the horn turns the lights on?

 

Just so we are clear.
1) Remove the TAIL relay
2) Remove the HORN relay
3) Put TAIL relay in HORN relay socket.
4) Blow the horn.
5) If the horn works - the TAIL relay is good.

 

Lol yea I was joking, but given my inexperience I guess that wasn't obvious. Ok so...
Tail relay is good. No power in slot 1. And both t-taps are removed and the wire is good...

 

Confirm that the WHOLE dash lights (switches) up and the console (speed.rpm.temp, etc) Dims when you select Parking position on the light select stalk. SO we know that the PANEL fuse is doing its thing...

 

Also adjust brightness UP n Down with the trip knob...