The cameras doesn´t work in Rbr. Only in Rfactor. Nothing to do about that unless your a coder..
... even if you're a coder, they would probably not work. The original rbr format (with working cameras) has completely different system.
But - for small go-around tracks you can build your track very low - around 0 meter in height and close to the 0,0 (xy) point - where yellow lines cross in 2D View - then you can have one more outside camera. See for example Kytt's Laitse Rally Park.
http://www.racedepartment.com/bobs-track-builder-wip-tracks/29897-laitserallypark.html