Is this the radio with 10 or so presets? There is a steel box in the left rear of the car ( at least on a R129 SL of the same era ) that is the other half of the radio.

Radios have 2 power feeds , one that never turns off to keep the memory going and the other switched power so the radio shuts off when you turn the key off. You might have lost the constant power.

Also, does this radio lose presets after you crank the car but not if you just cycle the key? If so, check battery voltage while cranking. I've had batteries with a bridged cell cause too much of a voltage drop and wipe the radio memory. Rather than a 12 V battery you end up with a 10 V battery that cranks reasonably strong and nearly as long as a good battery as opposed to a discharged / worn out battery with limited capacity.