Some day I'll start a thread here and go into the gory details of the writing/art commissions I get, especially after I started my DeviantArt page. Let me assure you, there are absolutely people who have vehicle fetishes (though usually they involve someone transforming into a vehicle, which is either less weird or way more so, depending how you look at it). Oddly, motorcycle transformation seems the most popular. I blame Stephen King; easily 90% of these people will mention "Christine" without prompting when they message me.
Actually, I could probably start a thread just going into all the weird, rare fetishes I've encountered and been offered money to create content for. Some of them are just bizarre.