Originally Posted by b.swamy
Nano does not have speakers on the doors.
It has a pair on the front dashboard and a pair on the rear.
Okay now I understood. The front speakers are above/beside the glovebox and rear ones are on the parcel tray.
Front speakers are 4 inch (not sure, you may check yourself with a scale) and rear ones are 6/6.5 inch round coaxials. Components are not easily available in 4" category, so you can keep the front speakers stock for now. For the rear, if the parcel tray is detachable, get 6x9 inch coaxials installed on a new MDF tray. That would be totally worth it.
6x9 speakers from JBL and Pioneer are good if you are on a budget. They will cost ~3.5-4k for a set with decent capabilities. Go to a nearby shop and audition these 2 brands, and decide there itself. Fix them on a MDF parcel tray (costs 800 to 1k) and you will have enough improvement as compared to the stock setup.
Getting 6/6.5" round coaxials on the rear stock tray will not give really good results because I think the stock ones are also the same size. Marginal improvement can be expected, nothing much.
Getting 6/6.5" components at the rear is also not advisable as tweeter placement becomes a problem. If you place the tweeters on B Pillar, they will interfere with front soundstage as they will be too close to your ear. Best option is to get 6x9 inch coaxials from some good brand. It would be totally worth it.
Do note that no additional wiring would be required. Ask if you have any other doubts.