Regardless of whether you use an additive, a car’s Diesel Particulate Filter (DPF) will still need to be cleaned (or regenerated) periodically as soot builds up in it. Short trips where the engine doesn’t get a chance to warm up properly, as well as extended periods of idling in traffic and stop-start running will all hasten this requirement.

In the case of the Captiva diesel, the best way to manually force a regeneration is to put aside an hour and go for a decent drive. The advice from Holden in the day was to travel at more than 50km/h and at more than 2000rpm (which may mean locking the car out of overdrive) for a minimum of 25 minutes. During this process, you should not allow the speed or revs to fall below those two figures which suggests finding a decent strip of freeway to carry out this process. You should also not turn off the engine at any point in this procedure.

The broad idea is to get the engine and exhaust hot enough for the filter to regenerate and clean itself. Experience suggests a couple of attempts may be needed to get this to happen as it should, and in fact, the car’s computer will give you several chances to produce the desired effect before the workshop beckons.

If, after 100km of this type of treatment, the DPF light on the dashboard hasn’t disappeared, the solution is a trip to a workshop to have the filter investigated and, potentially, hand-cleaned. Also, a mechanic can initiate a DPF regeneration via a scanning tool.

This looks like a classic case of a body computer that is in the process of dying. Any time you see random lights coming on, or the stereo switching itself on, suspicions are always that the body computer as the culprit.

This computer controls literally hundreds of functions from the wipers, climate control, lights, entertainment system, even stuff like the rear window demister and central locking. And when the computer starts to fail many of these functions can go haywire, which is pretty much what you’re seeing.

The tail-lights staying on and the stereo witching itself on are what’s draining your battery, but the body computer is very likely the root cause and probably needs to be replaced. But before you do that, try this: Disconnect the battery overnight and then reconnect it next morning. Sometimes (but not always) this swill reset the body computer and might stop it playing tricks. The fix might be temporary, but it’s definitely worth a shot.

This sounds like a fairly simple case of the engine setting up a harmonic vibration through the car. It’s not an uncommon thing and diesels are worse than petrol engines purely because they vibrate more, particularly at idle. By revving the engine slightly, you’re changing the frequency of the engine’s vibrations and moving them out of synch with what’s called the 'natural frequency’ of the rest of the car.

Why does it manifest at the steering column? Because the column is a long, flexible (in a micro sense) shaft that is far more prone to picking up vibrations than a short, more solid part of the car. Why is it happening now all of a sudden? Probably because there’s some wear in the car somewhere. Your call to change the engine mounts was a good one as these can wear and cause this very problem. But, equally, the wear could be in the exhaust system or one of the points where it attaches to the car, a heat shield, the torque converter, or maybe even a piece of the car’s sheet metal that has come loose and is moving slightly.

The quickest fix is to bump the idle one or two hundred rpm and see if that moves the engine out of the vibration zone at idle.