We have a factory fitted LPG on our vectra and its so slow at the bottom end you really can tell the difference between the LPG and petrol
its the one with the toggle switch as mentioned to swap between petrol and LPG
The extra LPG tank is where the spare wheel is meant to be and the spare wheel goes in the boot.
One thing to note though about the LPG is that is cheap because of some government thing keeping it cheap (no tax or something) but it is only meant to last until early 2010