If you have Photoshop or Elements you can use the shadows/highlights sliders, otherwise you'll have to do a bit of dodging and burning.
I would also start with John's suggestion, but you might want to consider masking out the exterior scene as the shadows slider will also elminate the shadows on the wagon, which is something you might not want.
If you were to reshoot this image, you could consider using a flash to correctly light the truck interior. The background is so far away, so the flash would not have any effect on the exterior part of the image.
You would have to set flash power manually. Your current settings would (ISO, aperture and shutter speed) would correctly expose the exterior, and the litting bit of light from a flash would brighten up the cab. This technique is called "dragging the shutter".
FWIW, I question the need to make the inside brighter...not much there to grab the attention.