I'm pretty sure this will cause problems without having a resistor and extra relay in place for the stock lights. The ecu reads the amount of draw on the circuit and if it falls below or above you will get flicker from your lights. That's why some guys with cheaper HID lights have flicker problems because there modules don't have the built in resistors to "fool" the ecu to make it think the lights are drawing the same amount of power.
Personally, i would want a seperate switch regardless.

 

i agree with the separate switch. all of the electrical addition on "her" are run directly from the battery so there is no chance of problems. i also agree with the computer noticing more draw on a curcuit. however, if you tie in the relay switch hot ( the wire that activates the relay) it is a very minimal voltage to switch the relay. it may notice, but i wouldnt think that minimal voltage would be enough to screw things. that being said i run a separate guage 10 wire from the battery into the cab to a fused block that i have 10-12 accessories that run off it. if you have several acc. a fused block is a good idea. cuts down on wires and easy to shut down for maintenance etc.