After reading these lyrics, I'm even more convinced that at least one one level this song is a coded reference for making love.
After reading these lyrics, I am also even more convinced that most of this song is written for a woman's voice {from a woman's point of view}.

Here's my interpretation a couple of other lines in that song.

"My black Cadillac,
it left me and i had to sell my sugar in fact,
Hit and run alright"

-snip-

GUEST 31 Mar 07 - 10:00 AM, what do you think is the meaning of those lines?

Imo, it means the woman had to go out hookin {prostituting-selling herself one short trick at a time to make some money since her man]her big black cadillac] done gone.

Guest, if I'm correct about the [imo not so hidden] meaning of this song, that doesn't negate your statement that "he's [Lightnin Hopkins] singing about an automobile. Lightnin' was astonishingly prolific and could improvise blues about people walking by in the street--or driving".

And in Blues songs in particular, there are many covert ways to refer to sex.

Not wanting to take the focus away from the big black cadillac song , but if you have a mind to, check out the meaning of the line "throw my jelly out the window" in Leadbelly's song "When I Was A Cowboy"

thread.cfm?threadid=3444#2005508
Subject: RE: Lyr Req: When I Was a Cowboy (Leadbelly)

In the context of those songs, "jelly"-"sugar" {same same}.