NYC using vehicle-mounted backscatter search
Story here. It's not an "x-ray," but more like radar. Same thing they use (to the limited extent it has "use") at many airports. I'd say there are some serious Fourth Amendment issues posed here (compare the Supreme Court case on using passive thermal detection to spot "grow houses" through walls).
The article suggests that it can see thru the sides of autos. If have fiberglass panels, I wouldn't be surprised. But since the entire idea is that it reflects off metal, it's seem to me that seeing through a metal car body would be impossible, absent an enormously powerful signal and some means to read the return in very fine detail, perhaps not even then.