How do you people think of a system to 'motorized' your troops?
Since motorization of infantry became a trend in the inter-war years, I think it's reasonable to include such feature in this mod. Basically the idea is to make your party a lot faster on the campaign map as to simulate their superior speed over foot soldiers (I think the speed should be slightly/significantly better than all-cavalry party). It doesn't require actual drivable vehicle since only mechanized infantry goes to battle with their armoured truck.
As to the implementation of this idea. IMO this may be done though, in the most simple way, though creating a truck item that increase path-finding skill greatly (or to set the skill to a particular level) when having it in inventory. The model of the player on the campaign map while carrying this item will become that of a truck as well. You may even have different model of trucks on the market varying in performances.
However this may be a bit unrealistic since it doesn't reflect the quantity of the vehicles needed per men, the maintenance of the vehicles or the chances of losing them. Unless there's a specific system made for it, such as making a hidden 'truck number' for players (that you may check in the camp menu) that's purchasable in cities for a certain price and capacity (like 1 trucks for 10 men). As long as the party size doesn't exceed the limits, player's party enjoy the advantage of 'motorization' (speed upgrade and truck model in campaign map, and perhaps an additional independent vehicle inventory accessible by camp menu) . Each vehicles have their maintenance cost that's deducted like troops wages. Trucks would be lost by chances if losing a skirmish (in in real life), and player may lost all of them if defeated completely and taken prisoner.