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While leaving it hooked up to power, have you tried to turn it off (longer press on the power button, until the display goes off, not just muting), then switch off the ignition and leave for a while?
Assuming it's a Blaupunkt, you've got several marques' worth of support, they (similar units) appeared on a number of VAG cars.
Not sure where you might check the code presently, the Georgian fellow who called himself "rdecode" was polite, affordable and quick some years ago.
I'd forgotten about him! Glad he's still going.
Have read that the more times you stuff up code entry, the longer it makes you wait for another crack. This is without proof, though - just hearsay.
Hi Sportiva
Find attached extract from manual on radio code lock out procedure.
Love how it's referred to as the 'secret code' ;D