How can I fix a SEAT CD player that wont switch off and drains the battery?

The CD player is the standard fitted to a SEAT Arosa. Labels say made in Japan, and assembled by Matsushita Communication Deutschland GmbH, and Ref No CX-DV1020L, also 7M7 035 119. The player stays on when the radio and ignition are switched off. The current drain flattens the battery in two or three days. This condition appeared suddenly and there were no early warning symptoms. The player works fine in every other way. Visual inspection of cabling and main circuit board shows no sign of burning, short circuit, etc.