I did have one that came with the van. Have thrown it own and bought a square one from Dick Smiths for 89.00. Just hook it up on the inside of the van as high as you can. Haven't tried it yet but Halfpint had one so I copied her idea. Hers works well.
We have one - haven't used it much yet but here at Texas we have better reception than at Wellington Point. Saw one working before we bought ours - had perfect reception for Grand Final weekend at Canungra.