I was watching "pimp my ride" last night and am curious......?

.......do the people know they're gonna be picked or are they genuinely surprised? like, how come it's them who always answer the door and not another family member?? also when do they film their video plea, is there a shortlist and they tape so many then pick or do they pick then film the VT once the person knows? these are just things i was thinking about when watching so im intrigued to know.