70's to 90's werewolf horror novel, witch turns a boy's brother into a werewolf who eventually kills her but keeps her corpse as he loved her

This was a book I’d found on my brother’s shelf, and he’s quite a bit older than me. We narrowed it down to probably being a book from the 70’s or 80’s.

I only remember a few scenes of this book, and countless Google searches using them have turned up nothing. The last time I read this book had to have been about seven or eight years ago.

The book starts with a man and a woman (who if I remember is his mistress) driving in a car. A large dog/wolf jumps out in front of the car or onto the hood (I can’t recall exactly) and causes the car to veer off of the road. The scene ends with a wooden post ramming right through the mistress’ head.

Another scene in the book involves the protagonist and his love interest trying to get to a house owned by a witch (or at least a woman who practices some form of witchcraft) who has turned a boy’s brother (who she was sleeping with regularly but there was no real love she had for him) into the titular werewolf. But there’s a traffic jam. In the scene it’s also to be noted that the house has been set on fire.

This brother had a younger brother who had contempt for him, and the fact that he was with this woman, and he eventually turns into a werewolf himself, after being bitten by his brother or turned into one by the witch.

Still another scene is of a bag boy or whatever from a grocery store being attacked and killed by the werewolf in a phone booth, and he voids his bowels as he dies.

Another detail about it is that the witch who turned the boy’s brother into a werewolf is eventually killed by the boy, and he keeps her body on a mattress in a cave somewhere, since he was in love with her.

A scene toward the end of the book involves a face off on a bridge by the protagonist and the boy, who I believe by this time is a werewolf himself.

It’s very disjointed, but I’ve been looking for years on the internet to try and find this book myself, but have had no luck.