Each of the rocker switches in the RSVR HANDSET circuit has three terminals. The terminals on either side are SIGNAL OUTPUTs and are wired to the ARDUINO MEGA as INPUTS. The central terminal is connected to 5V.
Each SIGNAL OUTPUT is connected to a PULLDOWN resistor and which is grounded. Each resistor is connected to the SIGNAL OUT of the switch and the SIGNAL IN of the ARDUINO from the same side of the PULLDOWN RESISTOR. The other side of the PULLDOWN is grounded – more or less visible in the image below.