Sometimes he had to lift on an old minibus colt to get to his office. He would sit on the back side of the vehicle. Although his eyes were frequently sleepy, his hands held the metal part of the car. Like a bat hung its toe on a branch.
I'd rewrite some parts of the above:
Sometimes he had to ride an old Colt minibus to his office. He sat outside, on the sloping back of the vehicle. Although he was frequently groggy and always sleepy, ...
For the “...” part, consider phrases like the following.
• as if they were magnets, his hands held him to the metal of the car
• he held on to the metal of the car, as if he were magnetized
• he held on to the car, just as a monkey holds on to its mother, asleep or awake
• leechlike, he held on to the car
• he held to the car as if it were a prayerbook, and his only salvation
• he held to the car as if it were his wallet, with his life savings inside
Edit: From Des' comment that “the author used the bat (not monkey) as a parable”, I gather that this relates to translation of work by some author other than Des, in which case it is not on to freely change the simile as I suggested above. Also, looking at Colt minibus pictures¹ it appears that if he (the subject) was sitting, then he was riding on top, and not on the back, as your translation says. Also, most Colts have near-vertical backs, instead of sloping backs.
If he is riding on the back face, he couldn't sit, but would have to stand, perhaps with toes on bumper and hands holding to a top rail. To hang like a bat on a branch, he'd have to be head-down, ie, with toes hooked over the rail, and hands on bumper, which is not particularly plausible, so the bat metaphor seems labored. But you might say
He rode outside, on the back face of the vehicle. Even when he was groggy or half-asleep, his hands curled tightly about the roof rail always held him up, like a bat sleeping while hanging by its toes from a branch.
¹Note, Colt has to be capitalized. Also, rather oddly, those Colt pictures also include a baby monkey holding on to its mother.
Edit 2: The kind of Mitsubishi Colt minibus I referred to is illustrated in the pictures below that are marked “minibus”. If the Colt in question is instead a pickup, as at center or at bottom right, the translation should say pickup instead of minibus, and should refer to sitting or standing on the platform or bed of the pickup.