Hot wire?

The problem seems to be twofold.
1-your existing power supply is a series circuit, and when you load it further, it is loosing volts.
2- Your turning a series circuit into a parallel circuit, and your volts drop.

Your going to need to run a new circuit wire from the power source to the switch to the lights.

 

Are they? I just installed 5 of those little tiny LED marker lights and don't recall if I kept them straight or not. I guess I must have as it would be habit. I think that there were coded with a black wire and white wire. These went into rubber grommets. They all worked just fine!

To the OP, the other fellows helping you know more than I, but I would think that wire size isn't' likely the problem, though I have no clue what is given the tests that you have made.

 

Yep. They are technically diodes and diodes only allow current flow in one direction. Hook them up backwards and they simply won't light up.

 

Thanks NorthernMechanic, I'll remember that!

 

Thinking a little more on OP's issue there is no way he should be getting 0.1 volts on the power wire if the original lights are working. He should be reading battery voltage before the original light. An incandescent bulb simply will not light at 0.1 volts.

Reading what @thejackal posted makes me wonder if OP is hooking the new light in line with the original light, which would cause an unwanted voltage drop. The additional light should be hooked to power and a seperate ground.

Also OP do you have your meter set to DC volts and not AC volts? AC volts the symbol will show a "V" with a wavy line and DC volts will show a "V" with a solid line and a dotted line under the solid one.

 

Are you sure you're not spliced into the the ground side of the original lights?

 

Connect your meter directly to the ground terminal on the battery, and then measure the voltage on the wire. I think you don't have a good ground reference.

 

Led do no do well with surg or inconsistent power! I have been down this road and it gets expensive, run a hot wire direct from the battery with an inline fuse problem solved

 

Not to be technical but the LED consumes 25 watts of power. That is less than a 2 amp draw with the truck putting out 14 volts running.