Land Rover Discovery - best set up

Hi, I am looking for some advice to get the best sound for my Land Rover Discovery 2. Currently have a Kenwood head unit, focal 4.5 inch and tweeters in front, Kickers 6.5's in rear doors and an underseat sub. Sounds good, but the Land Rover is a big car and effectively all the sound is coming from the front half of the car. Earlier discovery's had 2 speakers mounted high either side of the back door, behind the 3rd row of seats, and I recently got a pair of Infinity 6 x 9's and think it would be good to add them on right at the back. So at the weekend I joined the infinity's to the Kickers in the 2nd row, so I have 4 speakers running of the same outlet from the head unit. It does sound better, even though I have not mounted the Infinity speakers, they are just sitting on the 3rd row seats. The back of the car now sounds like a 'tunnel of sound' the car is filled with sound. My question is now that I have 6 speakers plus sub, how best to wire them up ? Is it a good idea to be wiring 4 speakers together ? I am not experienced at this, but thinking it might be time to get and amp, and if so, what one would I need, is it a 4 channel, and how best to wire up the 3 sets plus sub ? Any help will be hugely appreciated. Cheers, Matt.

you are correct in one aspect, its not really good, in general, to be running extra speakers off of existing speaker wires because it will tax the radio. the best thing if you want to keep that type of setup is to get an amp for any extra speakers. if you can tell us what year your land rover is and maybe some more details about exactly what you have already done it will help give you better advice.

Thank for the offe of help, I bought a Kenwood amp at the weekend, 4 channel, wired rear 4 speakers into 2 channels and Focals in the front to the other 2 channels. Sounds totally awesome, really happy with it. The only complaint is that there is a slight whirring that matches the car accelerating. Is there anything I can do to get rid of it. It was all wired up with a specific 4 channel amp wiring kit.