The Horch KFZ/17 used a screened ignition system and incorparated ballast resistors. Not all German vehicle types used ballast resistors, and only the radio vehicle versions would have been fully screened.
Here are some examples of screened ignition components, in this case for a Horch V8 engine. Only one of the two ignition coils is still attached, the Horch V8 they used two. In any case you can recogise the distributor and ignition coil to which two boxes are attached. The larger box is the RF filter unit, the smaller box contains the ballast resistor. You can see the thin wire coming out of the ballast resistor box, this would go to the ignition lock. All the wires downstream of the ballast resistor box were shielded (including the spark plug leads which have been replaced here):
Here you can see the components fitted to the firewall of a Horch Kfz 17. You can see several filter boxes between the ignition coils. The two "upright" units are RF filters for the ignition coils, the small boxes connected to them contain the ballast resistors (the larger filter and the filter further to the left are for the generator/regulator circuit).
hope this helps,
Funksammler