usual favourite is the heater valve on the bulkhead.To test it you need a noid light or a sidelight bulb holder which fits into the heater valve connector.When you select hot on the dash i think the light illuminates permanently,midway it pulses on and off and cold it stays off(or could it be the other way round for hot and cold?)
If that checks out ok an no heat is getting passed the valve to the matrix then chances are the valve is snookered as they are known for it
Gary
July 2006