You can use the small wire between the RCA's as an additional ground or as the remote. Personally I do neither and don't bother with it. I run the power cable from the batteryand the remote wire up one side of the car. The RCA up the centre. Use the same guage earth cable as your power cable and attach to a good earth in the boot. I find the best is to give the amp/amps there own unique earth rather than trying to share an existing one