The shift lock is the mechanism that keeps the gear shift stuck in PARK unless the ignition is on and the foot brake is depressed. There is also a manual override button, I guess intended for allowing the shift to neutral for towing etc even when the car is switched off.
For some time now, we’ve been resorting to the manual release button because the automatic mechanism doesn’t work. (2003 Nissan March)
The mechanism is pretty simple. Circuit diagram here: Micra K12 manual page 3912 on manuals.co . There is a solenoid (black/white wires) in series with the brake switch, and a microswitch (blue/white wire) that is closed when the shifter is in Park.
Getting access to the mechanism is a matter of removing two screws either side of the centre cover at the rear (push seats forward to access). Lifting the cover from the back will pop out the two clips at the front.
Using a continuity tester I found the microswitch wasn’t closing when the shifter was in park. However, pressing it more firmly caused it to close. Probably the plastic cam that presses the switch has finally worn too much.
I fixed this by moving the shift out of Park, and carefully bending the microswitch lever up towards the cam. The above photo shows it closed now.