Voltage Drop

Use your volt meter to measure the voltage drop IN the wire, by measuring the voltage in both ends of the same wire while the circuit is powered up. If you check it in sections, you'll soon find where the voltage is being dropped, by working in sequential sections down the wire.

This is different than checking resistance. Good voltage drop is 0.2 volts or so. The more drop you measure, the more is "lost" within the wire.