In May Swenson's poem "Southbound on the Freeway," the poet never really tells us if humans are the guts or the brains. Instead, the poet poses the question and frames that question within the poem, as an alien tourist from Orbitville mistakes cars for the creatures who live on the planet.
The personification of the automobiles is apparent as in the alien's perception "eyes" are headlights and taillights and "feet" are tires. Interestingly enough, however, the cars can only "glide, like inches, down the marked tapes" (stanzas 11-12). So even though automobiles can move, their movement or freedom is restricted by the roadways. This suggests the shadowy bodies inside are brains as humans have more freedom of movement than the automobile.
However, humans also depend on cars for transportation. And, if automobiles represent technology, then we could be the guts, dependent on technology.