Hi,you can connect an amplifier by either connecting to pre-outs featured on a head unit or, if the amplifier supports it, you can use a high level input (speaker wires) as an input.
As you mention he has taken a connection from the front speakers, I assume he then has disconnected the factory speaker wires from the front speakers and run you fresh speaker cables from the amplifier to the speakers?
That is the only way you could do this without snipping the speaker wires from the rear of the head unit and running those to the high level input of the amplifier, then doubling back the speaker cables and reconnecting them to the factory speaker wires.
I hope that is clear? I could provide a sketch if required
