Factory speakers on the st162s are 3.5" all round, hence the standard system sounding pretty crappy.
In the dash, below the factory speakers, there are pre-moulded holes for 4" speakers (maybe a factory option overseas?). A good idea if u want to install 4" splits in the front and keep things looking factory, is to make a bracket for the tweeters and place them in the spot where the 3.5" used to live.
If you want to install 6x9s in the rear best bet is to put them on the parcel tray, but 6x9s won't get u nearly as much bass as a decent sub.