Ok since u have already decided on the rear speakers.....I guess u are of the opinion that the rear speakers need to be the best speakers of the ICE.
Well, if you are driving the car yourself, you should focus on the front speakers. The rear speakers should just act as fillers, if at all you want to install rear speakers. You can completely do without them. In my Alto I dont have rear speakers, just front components, a 4 channel amp and a sub at the back.
So within 20k, this is what I can suggest
Front components: Illusion Audio 5.25" --- 5.5- 6k with bill
Pioneer 5050 HU --- 6.5k grey
2 channel Amp --- Sony/JBL --- 3.5k
Wiring (RCA + Power)-- 3k ---Ground Zero
Installation --- 1 - 1.5k
________
Total 20k
You can then later on add a sub with a mono amp or sell the 2 channel and get a 4 channel and power the front components and bridge the other 2 channels.
Get the installation done from Boom (Zohir), Carter Road Mumbai or FM Drive Khar, Mumbai (Pankaj bhai) ......
Boom is a franchise given out by Hermit Audio (B &T, team-bhp).