You are right there mate, as the LED bulbs run on lower power than standard bulbs already the resistance is low, you buy a LED bulb with the appropriate resistor in to compensate for this, but then when you tap into it your causing another drop in resistance which bring the light on on the dash.
Ive got a weird one at the minute with my MK5. Ive got LEDs in the sidelights, rear plate lights, front foglights and ive got HIDs in the dipped beam. Put my sidelights on on their own they work fine, put the fogs and sides on, works fine, but put the fogs, sides and HIDs on and both sidelight bulbs go out? It didnt do this at first so im hoping a new pair of bulbs solves it. Frigging canbus systems !