1998 Jaguar Vanden Plas heater not working

check to see if both heater hoses are hot when car is warm. if they are then check the servos under the dash,pic enclosed. it may be your water valve isn't working if both hoses are not hot.

How do you access this mechanism?

it'snot very easy best way is to look through from where the gas pedal is tosee ifyou can gain access there. make sure both your heater hoses are hot first though as it may be hte water valve on theengine not operating.