You will need to purchase 180 degree 5 pin female din that is chassis mountable.
This is the type that I purchased except mine also had a mounting plate too.
The standard 6128 cassette connector is wired like this.
Each pin is numbered on the back so that you know which one you are connecting it too so don't worry if the pins are in different positions as it depends on whether you are looking from the connection side or the soldering side.
I would use different coloured wiring for each type of connection. I used the following
Ground - Black
Data Input - White
Data Output - Red
Remote Control - Brown
At this point I added 4 male pins to the end of my wiring so that I could build a connector so that I could easily disconnect the DIN should I need to work on the cassette drive.
Next is connecting the wiring to the cassette drive Make sure that when you solder you do not create any shorts and connect the wiring as follows.
If you want to use the built in cassette recorder you just unplug the cassette leads from the DIN and use it as normal.