Joe, first of all, the only M8 i know of isnt a howitzer, its an armored car with im guessing a 37mm cannon. its got six wheels and an open top turret. much of the time, tanks and AFV's were painted white inside to help in illumination. ive seen an M8 or two, with the interior painted white like this. i cant say for halftracks or some german vehicles, but my guess there is that they were painted the general exterior color.
for vehicle antennas, i believe i use a size E. its wrapped in smaller wire, so you'll have to unwrap it, or simply cut it off. dont worry its not as difficult as it sounds. just unwrap what you'll need and that's it.
as for the german uniforms, in what theatre are we talking about here?? north africa? german camo patterns?? africa i think yu could get away with either a khaki or a sand color. the adverse affects of the heat and sun on the uniforms would fade the colors rather quickly, along with the dirt and dust of the desert.
i dont normally build modern AFV's so i cant help you witht he leopard bergepanzer question.
hope it helps. have fun...