The markers can easily be separated on the existing board with only 2 soldering joints. They can be switched on/off using 2 function buttons. However, you are restricted to having them either off or front white/rear red on or rear white/front red on. Or you can any other combo that can be wired using only 2 AUX tabs switching on/off using 2 function buttons.
I have worked out the wiring on the 422s and have a complete list of my findings but this is what you need to do for the marker lights.
1. Remove the 21 pin dummy plug.
2. Once removed you will see 2 pads marked AX1 and AX2 on the board. They output around 12.6V and can be mapped to function buttons. Let's say F5 and F6.
3. Front markers (No.1 End) remove the brown wire from RWK. Red Marker
4. Rear markers (No.2 End) remove the black wire from FRK. White Marker
5. Join them together and solder to AX2.
6. Front markers (No.1 End) remove the black wire from RRK. White Marker
7. Rear markers (No.2 End) remove the brown wire from FWK. Red Marker
8. Join them together and solder to AX1.
9. The wires attached to RWA and FWA are the marker light common.
10. Install your 21 pin DCC decoder.
11. Program AX1 to F5 and AX2 to F6 function buttons.
12. The markers now can be switched on/off and will work as I described above.
The resistors for the LEDs appear to be on the light boards in the wireless body in which I have not removed to check. My conclusion is because TFWK and TRWK pads output around 11.5V. The TWA is the common. These pads supply the voltage to the light boards in the wireless body therefore the 11.5V must be reduced on the light boards.
Hope this helps