This is what we have available for 99 to 04...
99-04 Mustang Vent Panels
Each vent panel can hold ONE 2-1/16" gauge or up to 3 switches. These panels pop in just like the stock vent registers do. The only thing that has to be modified is the bottom of the vent area (just behind the panel) where the gauge hits the plastic. The plastic needs to be trimmed or dimpled. We dimpled ours by heating it up with a heat gun and pushing it in. VERY EASY. You could cut it if you were ditching the heat on a race car. Switches are $6.50 each and that includes the custom engraving of your desired switch label.
1 panel with one Gauge hole - $25 plus shipping