The red Vauxhall Corsa was in the car park at Millennium House, in Lower Hall Lane, when it was spotted by a passerby at about 3.20pm.
A team from Walsall Fire Station used water to douse the flames.
Crew Commander Chris Hand said: "When we got there the dashboard's lights were on. There was also damage to the wiring under the bonnet. The lady owner had parked it 40 minutes earlier and had popped into town. We didn't need to break into it as she arrived at the same time as us. She was upset."
The RAC towed the vehicle away to check what caused the fault.