The radio is never actually totally "off". So although you may have switched it off, power is still applied to some of its' circuits until you cut the power by removing the ignition switch. The static is happening because some of the electronic parts inside the radio are failling ( capacitors ). You'll need to have the radio repaired or replaced with a modern unit. If you insist on keeping it all original,
www.beckerautosound.com does repairs.