Two thoughts. Is the lens cracked? Clear silicone might work in a pinch. Depending on where you live you could also get some 3M clear Bra. They use this stuff on everything, it's basically a transparent vinyl. Some sign shops may carry it, it can be ordered online, a few high end dealerships may carry it as well, also some of the higher end detail and accessory shops. It needs to be applied in warmer temperatures though to get a good stick, that's why I stress depends on where you live. Done right it should be fairly water tight, as when it's applied to paint properly it does not allow moisture to wick between it and the paint, if it did then it could cause issues with the car's finish. It would also be a far less noticeable repair than silicone on a lens face. Even clear silicone is going to show up and possibly distort the light pattern at night. You could also check for a light used with a nationwide salvage supplier like LKQ, or MPS Auto Salvage, although in my experience most Mustangs seem to be wrecked in the front, making those parts often useless. Keystone automotive might (I stress might) offer an aftermarket collision repair version of this light at a reduced price, but that's a long shot as well. IMO 3M clear bra would be the cheapest, and easiest thing to go with and what I would try first to save the $$. I would stress too that you need to resolve this soon, moisture in that HID system could (again I stress the word could because it's not a 100% certainty) cause other issues with the headlight system affecting the other light or all the exterior lights on the car, ending up costing you way more than the $700 part to repair in the long run.