IIRC, the later switch has four positions-
0: everything off
1: accessory position- radio etc will work
2: ignition- power to the fuel pump solenoid, glow plugs activate on the timer
3: start. fuel pump still powered, radio etc switch off
And you're converting from the older type with the separate glow-plug position, which works like this-
0: everything off
1: run- fuel pump etc powered, radio etc also powered
2: glow plugs- fuel pump powered, glow plugs powered, everything else off.
3: start- fuel pump, glow plugs, starter motor powered, everything else off.
So the new switch will have one input terminal- thick brown wire from the battery.
One terminal will be active in positions 1 and 2, but not 3. This is your accessory connection- white wires.
One terminal will be active in positions 2 and 3, but not 1. This is your 'ignition' circuit- white/black wire to stop solenoid and glow plug timer.
The other terminal will be active in position 3 only. This is the starter motor connection.
The best way would be to test the switch with a multimeter to identify which terminal is which.
I found a pinout diagram for the timer relay here:
Looks like you need a connection to the 'start' terminal on the switch too.
One exception I would make to the diagram is that, since you're converting from a wiring system with no 'accessory' position on the switch, pin 4 should go to the white/black wire connected to the stop solenoid, not one of the white wires. This means the glow plugs won't activate if you're just parked listening to the radio.
I'd install the relay behind the instrument panel, as then you can use the existing glow plug wiring that formerly went to the key switch.