Originally Posted by DaBigDogg
Basically from what I can tell, my 4157 bulbs (I don't have the VLED's yet, waiting for the right time to buy but understanding the theory) are dual filament bulbs. When the headlights are off, both filaments are on (high brightness). When the headlights come on, only the low brightness filament is active. When a turn signal is activated the high filament comes on to indicate the signal (low/high) versus when everything (parking and DRL's) is off in which case it's off/high. Without the dimmer wire to signal the control box to illuminate only to 50% power, there LED's will come on full power. What I'm saying is the difference between Canadian market and US market trucks is, with Canadian trucks, when you turn on your headlights, your DRL's turn off completely. With US trucks, the DRL's remain in a lower power state.
which is odd, because the manual specifically states that DTRLs are off when the the headlights are on. Nevertheless, your experience explains it, and there is no problem with the VLEDs then, they are operating as expected.
Perhaps they consider the one low output filament of the 4157 as a marker light, (and the hi filament is used as a signal/dtrl) which would be a change from the 2005-2011 models where only the weak outside corner light is used as a marker light up front.
Could test this by running the truck, turn on the DRL, then set the parking brake. DRL should go off (2005-2011 it will). If it does go off, then turn the headlight switch to Park, you should see the low filament, come on as a marker light then. That would confirm it.