G'day Leina,
For the reverse lamps, there is normally a black rubber plug that is fitted to the back of the tailight fitting (from inside the boot). The plugs get removed, and a "tail" is fitted. The tail has spring loaded contacts to supply power to the bulb, and electrical leads to connect to the wiring harness - see the green wired fitting in the photo below.
One way to get a tail is to butcher an old wiring harness, though care needs to be taken to ensure that the right type of bulb is used to match up to the contacts in the tail. Some of the tails that you can cut out of an FB/EK harness have two contacts, and hence need a bulb with two contacts - using a single contact bulb (like an indicator bulb) means that the bulb will not contact properly. Note that the tail in the photo above has only one contact.
Because the wiring harness I butchered only had twin-contact tails left, I ended up using a stop/brake bulb, as it has two contacts. Trouble is, the two-contact bulbs have their bayonet pins offset, and would not screw/bayonet into the light fitting. I resolved this by filing off one of the bayonets. A bit fiddly, but worked in the end.
Cheers,
Harv.
327 Chev EK wagon, original EK ute for Number 1 Daughter, an FB sedan meth monster project and a BB/MD grey motored FED.
... query though, is there another segment to the housing that is not normally supplied as standard to the rear light assembly..? auto elec just soldered the wire straight to the light bulb.. just checked on my old Blu ek and it too just had this.. ??? 
).
didn't think to look at the way brake indicator was set up! Will get the soldered connection replaced with terminals.. All good