No turn signal/brake light

[Kitsune474]

98 K1500 here

I bought a new cap for my truck and when I went to wire it in for the 3rd brake light I found that my drivers side brake light and turn signal wasn't working. The truck used to have a trailer light plug in the bumper before I removed it so there are a lot of extra wires going on but I can't find any wires that blink my test light when the turn signal is on. It's only the drivers side light too the passenger side works perfectly as it should. Any help finding the wire that controls drivers side blinker and brake light would be much appreciated.

 

[Kitsune474]

***UPDATE***

So I've made sure all the wires in the tail light harness have continuity, I assumed maybe a wire had been pinched somewhere along the frame. I disconnected the plug under the brake master cylinder and decided to check the male end that goes into the cab and alas no power for left brake/turn signal but there is for right. I'm stumped as to where to go from here.

 

[GoToGuy]

where are you getting power for bedcap third brake light?

 

[AK49BWL]

Doesn't matter now, he doesn't even have power leaving the cab. If you have right blinker at the firewall but no left, the break is either the multifunction switch in the column, or the yellow wire that runs between that switch and the firewall connector you're checking. You have power TO the multifunction switch hence the right side working. Do you have front blinkers both sides? I would have to assume you'd know if the fronts were out as well due to the switch because your cluster blinker indicators run on that same circuit.

 

[Kitsune474]

where are you getting power for bedcap third brake light?

Haven't wired it in yet because of the issues I ran into

 

[Kitsune474]

Doesn't matter now, he doesn't even have power leaving the cab. If you have right blinker at the firewall but no left, the break is either the multifunction switch in the column, or the yellow wire that runs between that switch and the firewall connector you're checking. You have power TO the multifunction switch hence the right side working. Do you have front blinkers both sides? I would have to assume you'd know if the fronts were out as well due to the switch because your cluster blinker indicators run on that same circuit.

Rear passenger side is working and both sides on the front are working. I actually replaced the multifunction switch last fall in a hope to fix my cruise control (to no avail). I didn't have enough daylight today to poke around under the dash with the tester, but I imagine the issue has to be between the connector in the engine bay and the relay block that is mounted to the firewall inside the cab because the flasher relay seems to be working fine as far as I can tell.

 

[AK49BWL]

It is, otherwise none of your other lights would be flashing either. Power goes to the flasher first, then the m/f switch, then directly to the lights. The connector in the engine bay: that yellow wire there is running directly from the m/f switch.

 

[Kitsune474]

It is, otherwise none of your other lights would be flashing either. Power goes to the flasher first, then the m/f switch, then directly to the lights. The connector in the engine bay: that yellow wire there is running directly from the m/f switch.

I'll have to get in there with the 12v test light and see if there's any power leaving the switch

 

[Hurlatron]

I'll have to get in there with the 12v test light and see if there's any power leaving the switch

Let me know how you make out. Mine is doing exactly the same thing. Right hand everything works. Left hand flasher is fast, and only front works. Left rear brake and signal out. 1990 k1500

 

[Kitsune474]

Well after putting it off until I got pulled over for it one night I finally dove into the multifunction switch and realized it was completely my mistake. I was having a lot of issues with the passlock system (fixed by getting a new ECU) but at the time I was told I should try grounding a yellow wire under that dash. Low and behold I grounded out the wrong one and that wire goes to left rear turn signal/brake light. I should just be glad it never caught fire but after putting all the wires back how they came originally everything works great!