HR-101 - ADJUSTING THE HEATER CONTROL VALVE
HR-102 - HEATER Description and Fresh Air Ventilation System
HR-102A - HEATER AIR SWITCH Repair
HR-102R - HEATER, Removing
HR-103 - HEATER VALVE MOUNT - Bolts or Studs?
HR-104 - HEATER CONTROL - Attachment to Dash
  MG-203 - HEATER NOISE (csm)
HR-105 - HEATER TAG NUMBERS
HR-106 - HEATER DATE STAMP
HR-107 - HEATER VALVE, Repair Or Replace?

HR-201 - os - HEATER VALVE IMPROVEMENTS - by Bob Muenchausen
HR-202 - os - MGB Heater Rebuild and Upgrade (easier on the MGA)
HR-203 - os - MGA Heater Rebuild - by Chad Pagel

Also see Electrical Function and Testing and in Electrical Restoration and Modifications.