It sounds like the additional lead length is adding resistance to the PIR circuitry. The circuit was designed to provide the PIR with the proper amount of power. When the 10" leads are added, you've added more resistance to the circuit, which means you have less amperage going to the PIR. What you may have to do is alter the resistors that are before the PIR to allow for the additional resistance caused by the 10" lead length. The more wire you have, the more resistance you've allowed in the circuit.
If you have access to a multimeter, you can find the resistance that is caused by the 10" lead length. Then you can adjust the circuit resistors to allow for this additional resistance.
Do you know what guage wire you're using for the 10" leads? If the wire is thick, you've added much more resistance than if you used thinner wire. You can try using a thinner wire for your leads. Be careful though, because we don't know the amperage going through the wire. High amperage through thin wire can cause overheating of the wire, or surrounding components. You may need to find how many amps are running through the wire, then chose the best wire guage for that amperage. You can check the amperage with the multimeter.
Would you possibly have the ability to post the schematic of the circuit so that we can help analyze?