Adding rear speakers to 1.2HTP

I am looking at options to add rear speakers to a 1.2 HTP. 

I am looking at either fitting them to the doors in the standard position, or alternatively on the c-pillar - there is a handy speaker size 'hole' in the body and easy routes for the cable down to the floor and under the carpet to the headunit.  Has anyone gone down this route? Are these 'holes' for speakers in a higher spec model?? Is there an alternative c-pillar trim which has a grill rather than being solid? (before I butcher the existing trim).

What are pins on the head unit for rear speakers? (it is an aftermarket unit, but plugs into the standard ISO plug on the loom, and has not had anything changed on it)