Partly depends what the number is on front of the stereo.
There is a master reset code of
891440 Suitable for P6802.P1701
and
588410 Suitable for N404
the process to follow is:
1, Press top of button "TUNE" with button 1 depressed, "----" will be displayed, now input any 3digit code and press button "SCAN" 3 seconds, "ERR" will be displayed. Repeat 10 times until "HELP" displayed.
2, Press button 1,4,6 & the switch button at one time, "SEC" re displays. Press top of button "TUNE" with button "1" depressed, and input the first 3digit code 891 when "----" displayed. After inputting first 3digit code, Press bottom of button "TUNE" with button "1" depressed, "----" re displays, input the last 3digit code 440 and press "SCAN" for 3 seconds until "OK" displayed. Now can be normally operated on reset.
The numbers to press are obviously as per the radio models mentioned before the procedure. If the number on your stereo isn't any of those above, then it won't work. The actual code set on your stereo is user defined when it is enabled, so there isn't a way of getting the actual code that has locked yours. If the previous owner didn't set it, and they don't know it, that is bad luck really. Some dealers set the code as the last 6 of the chassis number, so you could try that.
If all else fails, I do have two mk1 head units sat in my garagae still from when I had my old mk1.