In your subject, you mention that the radio loses its presets but in the
body of the post, you mention that the clock loses its presets.

It is usually a bad idea to cut connectors off of factory wire harnesses,
especially without first referring to a wiring diagram. Hopefully, you at
least saved the connector so you can replace it if necessary.

Electronic tuning radios and digital clocks installed by the factory
generally have 2 power sources.  1 is always hot to retain memory and 1 is
switched with the ignition.  If you are losing presets, it is because you
connected the wire to the clock or radio that should be always hot to a
switched circuit.  It should be easy to check with a volt/ohm meter to
determine whether a circuit is hot all the time or switched.

Also, when you hard wire an accessory, you should use insulated connectors
so that you can remove the accessory for service or remove it from a circuit
if it causes problems.