Originally posted by Frjeff Mine looks pretty much just like this when cap is on or when inside with cap off.
It is outside when bright that it is impossible to read the bottom line. I may have to return it.
Okay, so that establishes that there's nothing faulty with your camera, since it looks the same as the example photo with the lens cap on. So your problem is stray light getting into the viewfinder when you're shooting in bright light. Returning your camera and replacing it with another K10D won't solve that problem, as any other K10D will be pretty much the same.
Options:
Try an O-ME53 eyepiece, or one of the various generic rubber eyecup options that you can buy online that will form an almost light-tight seal around your eye.
Or accept that a DSLR that went out of production over ten years ago just won't have the same display brightness as a current camera, and try a more recent model.