Last year I fitted a bowthruster to the boat. A Vetus one with one control in the wheelhouse and another at the external helm. Thought a remote may be useful at times but a fairly expensive option from Vetus. The joysticks are connected to the motor by a 4 wire cable with each joystick having an in and and out plug so you can link in a "daisy chain". Checking the Vetus circuit diagram the 2 inner pins on the plugs are 12v+ and 12v-. As you move the joystick 12v+ is supplied to one of the outer pins to energise the main relay to activate the motor in one direction or the other.
Purchased off ebay a Winch Remote Control
Quite a simple wire up to the unused plug at the end of the chain as the remote controller has 4 wires (+ antenna) 12v+ and 12v- in. The 2 other wires output 12v+ depending on which button is pressed.