The filter is not a proable one in diesel vehicles, from the point of view that you propose, now in vehicles with electronic injection, either that it is found before the flow and generally it is thus Endia today.
To test the heater disconnects the heaters, connect a driver to the battery positive and one by one try them: if you have current consumption they are good, so that you do not confuse you connect an amperimeter in Seri, to know that they are not short -circuit. I recommend you: If when you start notes that guide strongly and the spark plugs are good, with sults the problem with your mechanic .