The 48-year-old was attacked at the Sunshine Grocers, Green Street, Paisley at 5pm on Friday June 15.
He was rushed to the town's Royal Alexandria Hospital and is still fighting for his life there.
Detectives are now keen to speak to the driver of a car who was outside the shop minutes after the attack, which they are treating as attempted murder.
She is described as in her 30s, with short brown hair. They also want to speak to a man seen walking from the shop carrying bread who appeared to chat to a man at a nearby phone box.
Det Ch Insp Duncan Sloan, who is leading the investigation, said: "I believe there are people still to come forward who hold information that may be key."