I assume you say you cant use an amp because you only have the one set of pre outs?
If this is the case then you could use another amp - its not ideal, but you can buy splitters, so that you can split the one set into two sets of pre outs for a second amp.
Alternatively (sp), if you really dont want to go the amp route, you would need to run new wires through the car from the back of the head unit to the speakers. This means you would need to work out which wires from the HU were the rear wires, cut them off the connection block (which plugs into the car) then use a new connector block to wire the new speakers up directly. This isnt really the ideal way to do it, but probably the easiest and cheapest.