I saw in Grant's book on slang (The Official Dictionary of Unofficial English) the word "bomber" defined as a noun meaning: an old delapidated automobile.
I have also seen this word used as a common slang term in rock-climbing. There it is an adjective meaning "solid and secure beyond doubt." It is derived from the term "bomb-proof", itself a slang term usually applied to trees, boulders, etc that may be used for anchors for protective equipment. One might say: "There's a bomber tree at the top of this route that can be used for top-roping."
I've heard "bomb" used to mean an old junker, but "bomber" is new to me.
Your post sent me off to look up "dilapidated." Interesting! American Heritage traces it to Latin dis- "apart" and lapidare "to throw stones." A dilapidated auto could certainly look like stones have been thrown at it.
There's another slang meaning of "bomber" that I like: it's a tagger or graffiti artist, especially one who puts his mark all over a town or location. I first ran into this term around 1997 or so, in Greece. From the metro system (the tube, el, subway) I could see a great deal of graffiti. One of the messages leapt out at me: "A religion called bombing." I decided it was an antiwar message but years later when I started reading up on graffiti slang (learning terms like all-city and kinging) I realized it was a tagger professing a love for his art.