Since you say you've selected the day mode, that means you also have parking or headlights on. When your lights are on, the backlight in the MD doesn't run at full power. All that the day mode does that I can see is tell the computer to draw the brownish background instead of leaving it black. As an experiment best done at night, turn your lights on, and set day-mode on the MD. Then turn your lights off. You'll see quite clearly that the "day-mode" is just the background, and that the display is still darkened when your lights are on.
I need to go play in my car again, but the instrumentation dimmer knob does have an effect on the MD when you turn it all the way up. I can't remember if it only sets day mode, or if it reverts to headlight off mode, but it definitely does something.
This is all based on my 2002, but I suspect 2003s behave the same way. I also concur with the previous poster's comment about cold weather. Until the MD warms up, it will be dimmer than normal. This is true of all LCD backlights I've seen; the flat-panel displays in my office are by the window, and are also dim on cold mornings.