Ames Nissan, of Fison Way, Thetford, admitted having glass in place that was not safety material and was unmarked, risking the health and safety of visitors.
Andrew Smith suffered severe cuts to both arms and his right leg last August and needed surgery lasting five hours, Central Norfolk magistrates were told on Tuesday.
Finance director Graham Balaam, who told the court the glass was already installed when they took over the building in 1988, said afterwards: "I hope this is a lesson to other dealerships in the county. They need to check they have the right glazing in all windows and partitions."
Ros Bedward, prosecuting for Breckland Council, told the Swaffham court Smith walked into clear glass near the showroom exit door and heard a loud crack as the glass shattered.
"Despite the fact Mr Smith's arms and legs had passed through the pane, he instinctively backed away and as he did so large shards of glass fell to the floor while others were left hanging in the partition frame.
"The glass was not made of a safety material or protected against breakage and no precautions were in place at the time of the incident to reduce the risk of the injury."
Staff members helped Smith. An air ambulance was called and he was taken to Norfolk and Norwich University Hospital.
Bedward said Smith was confined to a wheelchair and had both his hands in splints for many weeks afterwards. On returning home, he could not climb the stairs.
Judith Ayling, for Ames Nissan, said the company had taken steps to make sure a similar accident could not recur.
A full survey had been carried out by glass specialists, glazing replaced where necessary, and the company had fully reviewed its safety procedures.
The company was ordered to pay £5,000 costs.