Moored830:
All Mustangs with halogens have the same issue. The wiring to the headlights is pretty light gauge and the bulbs don't get the power they need to work at full output. Of course, if you headlights are not aimed properly then that could be a part of the problem, too.
May I suggest that you consider installing a Putco 239008HW wiring harness to get full voltage to the headlights? It is a 'universal' harness but fits our cars. The harness consists of a wiring harness to the battery, the main harness to the headlights, and two relays. It is plug and play (mostly). I had to extend the grounds for each headlight but that was trivial. I replaced the bright yellow plastic sheathing with the more common black split/corrugated wire loom for a more factory look. I have no pics of the lights on before and after but, trust me, the difference is substantial. The more expensive option is to go with an HID conversion kit (diodedynamics.com seems to have a very popular kit).
HTH,
Chris