Audio Systems: Head uint working, speakers not, wire pairs, factory speakers

Question
I have a 1996 bmw 318ti, i took out the old head unit because the cd stacker no longer works. I brought a new head unit with a cd player as part of the unit and installed it. The head unit seams to be working fine i can change radio stations the cd player playes e.t.c but no sound comes out of the speakers. i test all the speakers by running a short wire from the new head unit straight to each speaker (e.g. avoiding the cars pre existing wirering and they all work), but when i connect the head uint to the factory speakers via the cars existing wireing i get no sound.

Answer
Most likely there is an amp or speaker relay interfering.
The simple solution is run new speaker wire pairs, and abandon
the factory wiring. The techy solution is to buy a proper interface that will turn on the OEM amp(s). PIE, Peripheral, Scosche, and Metra all sell these products.

Kelly