The money was contained in two leather bags, padded out with about 60 plastic carrier bags and other rubbish, and had been jammed behind the front seats while the pair went shopping.
They parked their G-registered Datsun in Taunton's Victoria Gate car park, which is free, rather than pay 40p elsewhere. The car was unattended for about 20 minutes, during which time a window was smashed.
The man, 40, and his 68-year- old mother, live in a council house in Chard, Somerset. Police found the empty bags in council gardens near by and picked up pounds 3,000 in rolls of notes. The pair are not being named to prevent them being targeted by other thieves.
Det Sgt Brian Robb, of Taunton police, said: 'It is incredible someone should carry so much cash around with them and leave it in a car. The couple told us they don't trust banks. They . . .drove to Taunton police station to report the theft and showed no emotion - it was just as if you or I had lost a pounds 1 coin.'
He added: 'I asked the son how he knew how much money was stolen and he said he last counted it 12 months ago. It seems they just never spend any money and any spare cash is stuffed away in the bags. They've been carrying this money around for years.'