Dan, thats odd, my experience with your mod is that the collision box stops the car getting near a node to climb it. I raised the collision box and the car then can then float through and up the slope with the boat physics of PilzAdam's mod, i'm not sure adding step height has any effect.
Esteban, i looked at your code and noticed you have duplicated the original code for each car. there's a lot of unneccesary duplicated code, for example the helper functions only need be defined once, the remaining code should be edited for multiple car use ... although i don't know how