Most diesels of this era don't have a separate heater timer but just a glow plug relay powered from the ECU.

The ECU gets inputs about engine temperature then decides how long the glow plugs should be powered for.

You may have a temperature sensor fault giving incorrect info to the ECU or you may have a poor contact in the glow relay.

Put a volt meter on the glow plugs & check the voltage as soon as the ignition is switched on.