Is it best pratice to put in on the glass for reception purposes ?
Or can I hide it in the C pillar or even *behind* the trim of the hatch cover pieces of my SUV ?

no they need to be on the glass. hiding them will reduce receiption greatly

What about using a boomerang antenna like one of these:

http://qualitymobilevideo.com/Merchant2 ... de=TVABOOM

http://www.accele.com/accele2/Video_Accessories.htm