what is the purpose for the relay harness? and do i need it?
the relay harness bring power sources directly from battery to the ballast,(sometimes the flickering is due to the lak of power) and use the lights power sources to turn on or off,... so its a wire kit
it is not always require, it depend on the brand you get
the hidextra one you dont need the harness, just the capacitor,
but if you put them on, it will be one more insurance step to a flaw less system. also their not very expensive but it make the installation a bit longer (maybe 15 minutes mores)
the capacitor is similar to a fuse or a resistance,.. this is used to avoid flickering and the codes like tpms
it plug between the hid light and the hid ballast
so the capacitor, are require regarless of the brand you get, they are normally around 10$ for a pair