Just cut the car out and pasted that on to a new document for time being, Used a filter on the main image and then just brought the cut out car back onto the image covering the car which has been filtered, Theres better ways to do it but that was just quick and simple.
There should be a tool called "magic wand". You can change the tolarence levels of this.... if you increase it, it will select more of the image relating to the area you clicked on, eg if you on a black area and it adjacent to a grey area. This will highlight all the area, if the tolerence is set higher. You can just then click the delete button. If you want the picture transparent, then it will need to be in a GIF format.
Now if that made no sence I will write a tutorial when i go home. Sorry at work and trying to quickly think it through my head lol.
I'll try and ppost a couple of tutorials for Photoshop