I have a feeling that enclosure is going to be going across the entire width of the truck's seats. 1/2" MDF isn't going to like stretching that far. 5/8" should be alright as long as it's braced.
if you brace it its fine. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif this is for low power applactions btw.
and the real question is, how did you build a box that needs a 1/2 inch back to save space? the back and front walls are usually on the outside from the sides.
using 1/2" for the back would let me make the top and bottom depth pieces a 1/4" longer giving me a little extra airspace im working with very little space so every little bit helps