Corrosion, grime, paint, loose fasteners, and bad terminations are the main impediments of good grounds. Addressing those to the best of your ability will prolong proper electrical system function. It will involve some periodic maintenance on your part, as grounds tend not to exist in a hermetically sealed bubble. Chassis grounds are fastened to sheet metal at various points around the vehicle. The best grounds are those that are securely fastened in direct contact with pristine sheet metal, which can be kept entirely free of corrosion and grime buildup between the terminal and sheet metal contact. That's easier to accomplish with the ground for the instrument lights than it is for the rear light group. The under dash is generally protected from the elements, while the exterior light groups are susceptible to accumulated moisture and grime exposure, the building blocks of corrosion. Applications of dielectric grease or Permatex Battery Protect/Seal help keep the corrosion at bay.