First I’ll assume you’re running relatively recent versions, otherwise you’d probably be ranting about invincible solar-panel seeking bushes.
Currently collisions only directly hit “external” parts, which I think boils down to frames and wheels, this is problematic in that that means plating is specifically NOT included 
Everything else is hit by secondary “shock”, the frequency and intensity of which falls off exponentially with distance* from the impact.
Making it prefer to hit plating** if installed at that location is totally reasonable, and probably also reducing the shock effect if it does hit plating.
Ninja’ed: Yes vehicle mass makes a BIG difference, as the impact on you and your target are calculated in part based on relative masses. The bigger your vehicle is, the more impact on the target and the less impact on your vehicle.
*coordinate distance, not “through the network of parts” distance.
**By “plating” I mean “a part with a “reinforced” flag” or similar.