I thank you Hunterboy for your reply.
I found out that on my Aveo the reverse switch is located at the top of the gearbox. I replaced it but the problem remains!!! For now I have disconnected the switch connector while waiting to find a solution. I suspect that the defect involves the mechanism (inside the gearbox) that operates the switch. I have taken some photos that I will post in the forum for those who need to replace the switch.