Mercury - GPS Extension Cable 3.2-Foot CAN P Line 84-879171T21. Overview. Sometimes the factory cable on a GPS antenna just does not reach the spot where the signal is strongest. The Mercury 84-879171T21 Extension Cable gives you a little more room to work with-just over three feet-which is often exactly what you need to get the antenna into clean sky. It's made for boats running the 8M0024918 GPS Kit and carrying a CAN P signal, especially setups without an Auto Pilot module in the loop. Once i