1978 DT125. I am wanting to just add turn signals as the LH switch is there. Now, when I flick the signal lever for the left signal left and right come on. I disassembled the LH switch thinking that the selector that slides in there was shorting to both. I totally cleaned and inspected the slide part but it only contacts either the center and left or center and right. . Even if I place the test light on the slider, both sides still come on. It's as if there is something back feeding this. 
I am hoping someone has has this issue and can describe the solution. 

What are you using for bulbs?

Check your colors in the headlamp. There's dark brown & light brown.
Also i think there's Dark Green & light green/.
So left blinker goes to Dark Brown & Right to Dark Green.
Other wire is earth black for both blinkers.
Rear ones are the same.
You did well to start playing in the switch contacts without things jumping out & hiding under the bench. Hate that job.

No actual turn signals hooked up yet. I don't get why with key on and power is to the center lug on the actual switch. Switch to left and the left and right have power to the test light, the switch contact is clearly only contacting the center and left. Same as the rh position.

 

As RT325 suggested, just make absolutely sure that, in the headlight bucket, dark brown is going to dark brown and dark green to dark green. It is likely you have a triple female connector of each colour coming from the main harness into the bucket, and these are filled by the corresponding wires from the indicator switch, front indicators and flasher warning light.

I will check that.

Looks like i jumped the gun & misunderstood your post.
Hope you get them sorted. Just hook them up & see what happens,
On second thoughts, When you say they're both live at the same time are you talking Dark-green & Dark-brown out of the switch.
I guess if those two are live then there's something mysterious going on.
Not sure now what's in those switches, but do they have a fiber plate inside the copper bridging piece?.
if that was missing what would that cause, would it short to earth through the spring & ball, & if so what would that do.
Not sure, & now i'm woffling on.

The turn signal indicator bulb in the gauge grounds through the indicator lamp in the gauge.  It's an ingenious way to get the pilot light to flash with minimum wiring.  The little bulb in gauge cannot pass enough current to make the other side light up so if you use stock bulbs it works fine.  With LED bulbs or a little test light you'll see what you're seeing.

Bottom line:  Nothing is wrong. 

Oh, and if you want to test what I'm telling you about it being normal and expected behavior, simply remove the little indicator bulb in the gauge.  Now your test light won't show power to both sides anymore.  That little bulb is where the connection is being made. 

It's an unusual arrangement but works fine in practice. 

I've repaired quite a lot of these old switches and test the circuitry by attaching my test lamp, via a battery, to the brown/white wire terminal and each of the dark brown and dark green in turn. When working correctly, each side only lights up when the switch is in that position.