Yes, you are indeed looking at an article on our Project '66. This little project started back in 1999 when I was the technical editor for Mustang Monthly. The car was to be restored in honor of the '66 hardtop I drove in high school. With a few liberties taken (A/C, stereo, etc.), the Mustang would be a duplicate of the car I once owned.
During the first two years, we made amazing progress, with popular articles on V-8 conversions, Deluxe Interior conversions, metal and paint work, and lots more. Then we hit a wall when I left Mustang Monthly for our sister publication, 5.0 Mustang & Super Fords. The freshly painted '66 was rolled into the garage and covered up. But there are a few stories left in this old girl before she makes the trek to the 40th Anniversary Celebration in April.
Deluxe seatbelts came with the option for a seatbelt warning light. My original Mustang had it, and so would this one. We found an excellent example on eBay and snatched it up. Along with the seatbelt warning light, we wanted to deck out the car with a few dealer options for the sake of show conversation. We called Max and Lisa Kotlowski of AMK Products, and plunked down some hard-earned cash for an N.O.S. parking-brake warning-light kit, an exterior lights-on warning buzzer, and an interior vanity mirror. Here's how they install.
3 Once the cluster was disconnected and the speedometer cable unscrewed from the back of the speedometer head, the cluster could be set aside for working room. | 
4 The seatbelt warning light was a factory option, so you won't find an instruction sheet or "N.O.S in the box" for purchase. Your only recourse is to find one on a parts car or buy it from an auction or swap meet. This example, found on eBay, had perfect chrome and a legible and secure red lens in it. The wiring was in excellent shape and the relay, though incorrect for '66 (who's going to see it?) works. And just because the item is N.O.S. doesn't mean it will work. Sometimes bulbs break in handling and parts fail with age, so be sure to test everything. We tested the seatbelt warning light using a bench battery and alligator-clip leads. The parking-brake warning light uses a rare bulb that has a built-in flasher, so make sure that works too. | 
5 We obtained the parking-brake warning-light kit and lights-on warning buzzer from the vast collection of N.O.S. parts from AMK Products. These two items came in their original boxes with complete instructions. |
6 Mounting the warning lights required drilling a .68-inch hole, which is within a breath of 11/16-inch, for each one. We used the instruction sheet from the parking-brake-light kit to measure and drill both holes. The instructions called for drawing a centerline between the two knobs, then measuring down the dash 2.31 inches. | 
7 For a hole such as this, a Unibit is a good tool to have. The Unibit easily starts the hole, and you can simply cut into the dash "step by step" with the Unibit until you get to the proper size of 11/16 inch. | 
8 With the hole(s) drilled, slip the lamp body into place for a test fit. The lamp body has a small bump at the bottom for orientation to the dash. We made a small notch in the hole at the 6 o'clock position to accept this. |