850 aftermarket stereo NO SOUND PLZ HELP!

Please help,

I have recently purchased a Volvo 850 Saloon

I replaced the stereo with a Pioneer aftermarket single din and all the speakers in the doors. All worked perfectly with the sub etc. I then decided to replace the deck speakers under the rear window. Since then I have no sound from any speaker apart from the sub in the boot.

I have checked every fuse and are all ok! The stereo and the harness work fine in my 940..... But nothing in the 850? What could be the issue?

My 850 doesn't appear to have any sort of standard amp.... But as stated the aftermarket stereo worked fine in the 850 before trying the deck speakers.

Please help me. Any advice appreciated.