They run in parallel, the loom plugs into the existing sidelight holder in place of the normal bulb to power two sidelights instead.
One of these is an LED which you then fit in the headlight, the other one you tuck out of the way - i also made a little cage to try to stop the old bulb from touching stuff that may melt
The holders i bought are these on ebay
http://www.ebay.co.uk/itm/181178128998
Though they seem to have sold out now