FWIW, I had a similar problem with some steel re-enforced concrete. I used a stone blade to cut away a load of the concrete, exposing a reasonably large amount of the steel, then switched to a metal blade to cut the steel. I did try a (new) hacksaw, but found it tedious, and seemed to dull the blade very quickly whilst doing relatively little to the steel.
In this case, I'd say use the hacksaw to cut away a chunk of the plastic/wood and then grind through the steel - but take suitable precautions to avoid sparks on the plastic or plastic on the grinder. Avoiding heat is probably the best advice, so moving the grinder about so that you're grinding different bits of metal at a time, and maybe just go at it for a couple of minutes, then leave it for 10 minutes before you go back and do a bit more.
Another answer recommends a sledge hammer. You may find that when stripped of the plastic and wood, the steel buckles and bends more easily than you might imagine (when suitably persuaded by a hammer). The problem is that's a one-way trip. If you start doing it that way, you're going to have to finish. At least with the grinder you could change your mind and switch to a different method.
The last thing I'll just add is that our family car has some Tardis-like qualities when it really needs to (ie. when I've bought a load of materials without thinking about how I'm going to get them home). You may find you can fit more in your car than you imagine if you try hard enough, so may need a lot less cutting than it may appear.