you install a load resistor across each light (power and ground) line, in parallel to the LED, to increase the current load pulled so the car senses that load and dosent think a light is out.... LED's dont draw enough current on their own and the car thinks a light has burnt out..... like my drivers brake light just did.....

I have the LED's on my license and like them.... remember that the load resistors get very warm..even hot....so place the load resistors where they wont hurt anything and can dissipate the heat....