If those aren't bright enough then I don't know what is going to be, LED's don't put out as much light as you think, being tiny little light sources it's not one big bright bulb. I would think they would be brighter at least than the old bulb, unless it was a new one. You didn't exactly get the best quality ones either, really.
The best I can say is here.
http://www.carid.com/1997-ford-mustang-led-lights/lumen-led-lights-10287631.html
There is a bulb called PlaZma, which is supposed to be the brightest, and you can get it in different colors if you fancy.
The product description,
"PlaZma Series Interior Dome Light Replacement LED Bulb by Lumen®, 1 Bulb. Offers twice the brilliance of standard LEDs using the same power, and features a lifespan 25 times longer than incandescents at a lower operating temperature."
Mine is pretty bright, but it's in amber so it's not going to be as bright as white or a lighter color.