It seems that Dell in their wisdom have a 3 connection barrell connector. The third connection looks like it is a signal from the charger to tell the laptop that it is the right one. I searched for ages to get one that worked. I paid a premium for it too, but it was the only one that was guaranteed to charge the laptop in the dock from the car.
That being said. My Inspiron in the dock will not turn on without the charger being plugged in. Unfortunately that is the way Dell does things. Unless you can work out what the signal is, you will more than likely have to buy a in car charger that works to get it to turn on while docked.