let me add to tiets way to doing it... the more "neater" way ;)
you melt all the plastic backing of the other side of the housing, solder the 2 wires on and then solder the other end on the loom of the tail light. (to see which one to solder to refer to the beginning of this guide on page 1.)
should look like this
to fit the bulb into the socket, just file off one of the earthing pins, when holding the bulb upright, file the lower pin, this way u can still use the other pin as a earth for both bulbs.
back behind the housing
that is all :)