I want to do some electrical mods like adding a push/pull switch to my truck' tailgate to wire into my door ajar light to let me know if it is open along with splicing an electrical tailgate lock into the door auto lock mechanisms.

To do this I need to test for which wires are active during these certain things. I do not have a lot of experience with electrical.

Do I need a full multimeter with piercing probes, or is it enough to get a non-contact low voltage test light to test the individual wires in the harness for when the lock is pressed etc?

  • I suppose it depends on if you want to know the voltage on the line. Commented Mar 8, 2016 at 2:03
  • It depends, i don't know if the other wires have power when the car is off. So if I see none of have power, except one when pushing the lock button, i suppose that should do it. The problem becomes when the other wires in the harness have power regardless of the car being off...then it would be hard to know which wire is wired to certain things. Im not sure..
    – Gab
    Commented Mar 8, 2016 at 2:06
  • You can have power and low voltage resulting in issues operating various devices that require 12v Commented Mar 8, 2016 at 2:35
  • What is a non-contact low voltage test light ? I never heard about this, an the only tools I can think of are more expensive than a simple multimeter. Btw: If you don't have piecing probes, simple needles might help.
    – sweber
    Commented Mar 8, 2016 at 7:02
  • @sweber I think it means a probe like the ones used to detect the phase by electricians... but for lower voltages Commented Mar 8, 2016 at 14:20

1 Answer 1


This is quite easy, but you need to do yourself a favor and get a factory wiring diagram. It will save you hours of frustration and confusion. The factory wiring diagrams will tell you exactly when the circuits are active and when they aren't. On top of that they will tell you the colors and sometimes the current flow.

You really don't want to hook up a tail light to a fan circuit (Personal experience) =P

Once you have that, you'll need some thin wire probes or insulation piercing probes. They come with the Power Probe 3, and fluke also sells them. A decent multi-meter should help you track down the wires you want to use. If you decide that you want to use your own method to pierce the wires, just remember to wrap them in high temp electrical tape. When insulation gets compromised the entire wire inside it can corrode. It's a real pain to try and figure that out.

If you want to try to sort this out the oldschool way you can use the multi-meter, probes, and a test light. Essentially what you would do is attach your test light or multi-meter to to ground, activate the circuit and test each wire individually. BUT I don't recommend doing this. As I said before, if you put a lock mechanism on a fan circuit that might intermittently kick on, you'll have some serious issues. You could possibly even put it on an EVAP circuit, which would probably mess with the EVAP without realizing it and every time you put gas in your car, your tailgate would drop. I would get wiring diagrams, there are so many wires in a vehicles it's really hard to do it the oldschool way.

http://www.eautorepair.net/ That website carries a lot of factory wiring diagrams.

I hope I answered your question!

  • Very informative, from here all I really am curious about now is whether something like the klein ncvt3 non contact voltage tester would suffice or would i need a multimeter that shows me voltage amounts?
    – Gab
    Commented Mar 8, 2016 at 18:56
  • Klein ncvt3 will not measure that current, see comments to your question. And you need to know the voltage per se for what you are doing. Commented Mar 8, 2016 at 19:41
  • @ErikvanDoren ok thx, I was going to blindly catch a wire to splice in. What would you recommend otherwise? Should a shell out a few hundred and buy a multimeter?
    – Gab
    Commented Mar 8, 2016 at 20:16
  • @Gab, plenty of inexpensive multimeters around, a few bucks not hundreds, something like a Jameco BenchPro 1562 or comparable will be enough for your needs and its like 10$, a few bucks more will get u one that has a continuity beeper if you dont want to bother looking at the screen. Beside that, the answer above told you what to do. Commented Mar 8, 2016 at 20:28
  • @ErikvanDoren awesome thx guys
    – Gab
    Commented Mar 8, 2016 at 20:33

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