12vDC lights come on for two reasons, they have a Ground and they have power, thats the "normal" reason, lol.
Second reason is because another circuit(usually another light) is using that one light as a Ground.
When that happens power flows thru the light and it comes on.
(any one who has hooked up trailer lights has run into this, lol.)
In your case I would check the grounds on the Head lights and parking lights for good grounds.
The turn signal light itself is probably not the issue, the issue is that it has a good ground and another light doesn't.
Because of the flickering with movement I would suspect a loose ground wire, but there could be a 12volt headlight or parking light wire that has rubbed thru with the turn signal 12v wire which could cause the same symptom.
The turn signal bulb is your "test light", so if possible have someone watch that light while you move wires around to see if you can find a wiring location that causes the light to come on
Last edited by RonD; 08-31-2014 at 04:38 PM.