I have a 1985 Honda Civic which I installed a Sony CDX-GT260 Car Stereo 50Wx4. I hooked up 2 Kenwood speakers sitting in oval boxes on the parcel tray above back seat in addition to the 2 in front doors. Sound is OK but I wish it was a but "fuller" with a richer base. Someone said fit an amplifier under the seat. Will this help and / or other guidance please.

First of all please check if the speakers are connected in phase. That is if all the positive and negative terminals are connected OK. Simple way to check is to turn the fader control first to the rear and then while playing some music with heavy beat ( dance music ) turn the balance to one side. Listen and see if you are getting good bass. Then turn the balance and bring it to the middle ( 0 ), when you do that if the bass is becoming flat-- one of your back speakers are out of phase and canceling bass. If thery are in phase the bass should become full and rich. Do the same by turning the fader to the front and try swinging the balance. If all is Ok , yes you will need an Amplifier to satisfy you. Make sure you buy something that can match the power of the speakers you have installed in the car.
The out of phase syndrome is a very common problem.