Speaker Balance Inconsistency

Morning all

Strange thing I've noticed the stereo in my '99 Seicento Sporting doing (its the standard Sony head unit that came with all Seicento Sportings). When I alternate the balance from left to right, there is a difference. When the sound is all-the-way to the left (passenger side) the volume is high and obeys the front/rear balance that I've set it to (60% rear). But when I tell it to come all out of the right (drivers) side, I only get sound from the front speaker. When the sound is balances 50:50 then both rear speakers work, but the further toward the right to move the sound, the quieter the rear right-hand speaker gets. It's so odd.

The rear speakers are after market ones cut into the parcel shelf. I didn't fit them and am not sure what brand they are (it wasn't well known. The stickers fell off and I can't remember).

Any ideas?

Cheers