The only thing normally with taging extra lights (loads) on to the existing circuits, is that it can affect the repetition rate of the indicators. That is when in making a "fits all" electric towing kit, its normal to include a double relay module so that the extra load on the indicator circuit is reduced down to next to nothing - ie a coil winding instead of an extra bulb. For Passat B5for instance, a different indicator assembly is available to handle the extra load without altering the flashing rate - which is defined by the DOT or MOT people.