Shorted Communication Line Test

Remove the driver's multiplex control unit from the driver's under-dash fuse/relay box.
Carefully pry off the driver's door switch trim (see step 5 in FRONT DOOR PANEL REMOVAL/INSTALLATION )
Disconnect door multiplex connector A (20P).
Disconnect passenger's multiplex control unit connector B (22P).
Check for continuity between No. 15 terminal of door multiplex connector A (20P) and body ground.
DOOR MULTIPLEX CONTROL UNIT CONNECTOR A (20P)

Fig 1: Checking For Continuity Between No. 15 Terminal Of Door Multiplex Connector A (20P) And Body Ground
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there continuity?

YES - Repair short to ground in the wire between door multiplex control unit connector and body ground.

NO - Go to step 6.
Check for continuity between B9 terminal of passenger's multiplex connector B (22P) and body ground.
PASSENGER'S MULTIPLEX CONTROL UNIT CONNECTOR B (22P)

Fig 2: Checking For Continuity Between B9 Terminal Of Passenger's Multiplex Connector B (22P) And Body Ground
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there continuity?

YES - Repair short to ground in the wire between passenger's multiplex control unit connector and driver's multiplex control unit connector.

NO - Replace driver's multiplex control unit and recheck the symptom.