I am searching for a book which I read in the early 90s about a man whose life has fallen apart. He gets swept up in a series of events which culminate in Ragnarok. The concept of the novel is the question of what's good and what is evil. I remember the Devil drives a Cadillac. Loki is also a character in the book. The man ends up choosing hell instead of heaven because heaven is too judgemental and rigid, while hell is pleasant.
I THINK you are talking about Job: A Comedy of Justice by Robert A. Heinlein, published in 1984.
The man's life isn't exactly falling apart.. but it seems that way to him, as he finds himself in what looks like his world, with everyone recognizing him.. but he has a different name, and the history and morals of the world are very different from what he remembers. And he falls in love with his stewardess (Margrethe.) Did I mention he was on a Cruise when this started? He had tried a fire-walk at a 'native village'... and that's when everything changed. He spends a lot of the story trying to take his new love back to his home in Kansas - not a very well thought out plan, as he's married in his home universe, and his wife is there.
Sudden 'World Changes' occur, usually at the most frustrating time; from his perception, it appears he's in the same world, but with different history, technology, morals, etc. Sometimes it's small (like when he's asleep at a hotel, and it just costs him his room), at least one time, it's while he and Marga are bathing in a river, resulting in them losing everything they have, except for the Razor that was being used to shave him.
Ragnarok (as his wife calls it; she follows Odin and the Norse gods), does occur; it's timed in such a way as to cause him to lose her, though. (He was too close to 'Winning' by converting Marga to Christianity, and would have been with her in Heaven; Armageddon is called just before that can happen.... JUST to keep his God from losing his bet.)
The devil is a significant character, and does drive a nice car (very futuristic; swivel seats, auto-drive, etc.) and, in the character he plays when our Hero meets him first, a Cadillac dealer. Loki turns out to have been the primary antagonist behind the scenes, in the end, although you could argue that the Christian God was actually the villain, as he made the bet that resulted in Loki's actions, and is portrayed as a bit of a brat.
In the end, after being 'Raptured' to Heaven (very nice.. but very dull), choosing to fall to Hell (much better than expected; it's had bad PR over the centuries), and meeting the being behind everything (so far as they know; they admit there might be more levels that the Gods don't know about) he chooses to be with the woman he loves, no matter what -- his memory of the supernatural events is basically wiped, and he ends up living happily in Hell, with his wife, kids, and many characters from the story, all playing different parts.
It ends with a beautifully ironic last line; he's now living happily in Hell (unbeknownst to him) and ends the story saying "Heaven is where Margrethe is."