Installed LED light bulbs in front blinkers and back brake lights. At same time replaced rusty socket. Lots of issues after such as no side markers, blinkers didn't work when headlights on. Turns out that the side markers get their ground from the dormant major circuit in an older style 1157 bulb and guess what? The LED 1157 bulb does NOT have the dormant major circuit. Long story short, I put the old style 1157's back (left LED ELSEWHERE) and all is well.
I ran into the same problem: http://mmb.maverick.to/threads/flat-panel-led-front-turn-signals.93416/#post-954554 Solved it by having Cougar's Unlimited make a special plug in part that allows a path for ground which the LEDs do not allow. He sells them on his site: http://www.thuntek.net/cougars_unlimited/default.htm You can see in my avatar how bright my turn signal running lights are. There is a second panel in there for the turn signal. Side markers are also much brighter than stock. Micah
