I really don't understand why people are saying pull it out and check the wiring.
the easy investigation (involving no tools to extract the unit, or risk of wires dropping out if the wiring has been patched in and secured with sellotape, like one of mine was) is to decide whether the unit retains its radio station memory. this involves no risk of the fuse blowing if the permanent live drops out and shorts to chassis, no need to patch in any other wires that have been bodged in, no trip to get extractors, no trip to the car radio shop, and no fee for a possibly nugatory "investigation" by some guy who is paid an (exhorbitant) hourly rate.
if it retains its radio station, its not to do with the wiring to the memory connection, and the reason it resets to the start is because its not designed to return to the last known track.