How about a popup of incompatible parts? For example: this exhaust is not compatible with towed hook, selecting the exhaust will remove the part: "towed hook" The list is endless, feel free to add your most annoying incompatibilities!
Not a bad idea, especially for parts like the bumper-mounted and tailgate-mounted spare tire holders on the hopper. Equipping both of those means you have 2 instances of the spare tire slot, resulting in a game crash.
Where is the best place to send a list of conflicting parts? Is it worth posting them here? Anything related to the game crashing is a complete disaster, but don't overlook cosmetic issues like the photo above! I don't think it should be like that when a part goes through another part, especially in the most visible place, this also applies to things like the exhaust through the hood etc.
Having 2 instances of the same slot does not cause a game crash, having infinitely repeating slots does. And the Hopper spares are not the same slot either, you just have 2 clipping spare tires.
