Installing a cd player in car problem

hi,i'm trying to install an after market sony cd player in a honda ascot.
i have haven't had the car long and bought it with a not going factory radio and cd player.
the factory unit would light up as going,but could only be heard playing out of the tweeter speaker on the drivers door.
so i removed and replaced with an after market sony unit.
same thing happens,
plays but can only be heard out drivers door tweeter speaker.
the car has two rear speakers and two front door speakers,
that are wired up.
would any smart cookies out there know what the problem could be!
i have installed car stereo's before with no problems like this.
i'm wondering if the speaker wires go though some kind of box or unit on there way to the headunit!
if so does any one know where it would be!
looks like a major to track the wires from speaker to headunit,
but that maybe the only way.
any help would be greatly appreciated.

What speakers are in the front doors. Aftermarket or factory!
Have you tested for any speaker shorts!
How do you know the speakers are wired up! Did you test them! Seeing wires attached to the speaker terminals is not a definitive way to tell the speakers are connected.
There is absolutely no need to trace wiring though the car to find this problem.

Try running wires directly from the head unit the to offending speakers to rule them out.
You dont need to put them in neatly at this point, just pull the unit out and temporarily hook them up to rule out the issue.
Have you actually removed the speakers to make sure they are connected correctly and securely!

I had a honda rafaga once that had a factory stereo so replaced it thinking it was the headunit that was faulty as i had a similar issue with only one speaker working despite all of them being wired ok, turns out there was this box thing that was broken (think it was some sort of sound enhancer/amplifier) either hanging under the parcel tray in the boot or under the passenger seat (have forgotten now) replaced with one from pickapart. all was good again. its probably better to just bypass it though with new wires :)

That sounds a lot like a factory amp. which I've never seen on that type of car before (but I'm not saying manufacturers don't like throwing curve balls too).
If that was a factory amp, it should be bypassed when an aftermarket stereo is installed.

Crossover perhaps!.

Could be any of the above - factory amp, crossovers (sometimes hidden in the doors), could be factory wiring that's been disconnected by the previous owner or connected to one of the above devices which is now non-functional etc.
The only way to do this properly is pull everything apart and see where the cables go (unless you have access to a tone tracer).
You say that the rear speakers are 'wired up' - but wired up to where! Do the outputs of your new head-deck connect to the same colour wires that are connected to the rear speakers!
It's also possible that these and the ones in the front are blown - hence no sound.
Do you have a multimeter! You need to test the speakers and make sure the voice coils are still intact. Otherwise you will be chasing your tail for days and get nowhere.

They Black box hanging from the parcel tray in my old Accord was the aerial power/controller (the aerial was in the rear screen)