The body of Somchai Neonthong, 50, was found next to a Toyota pick-up truck parked near a temple in Thung Tako district.
Somchai, who also worked as an electrician, had been shot at close range judging by the wound, police said.
A passerby had alerted police about the pickup that had been left with its engine running and the man inside, thinking there had been an accident.
Police believe somebody stopped the vehicle and shot Somchai as he tried to get out of the car. Police also believe the killer may have been a familiar person and assume the reason may be related to a conflict over a woman or business.