New Headlights, Low Beams not working, Hi-Beams half working

The most logical place for a failure that would create the reported symptoms is in the multi-function switch.

But before replacing it would be handy to run a few tests. Check for power into and out of fuse F2.4 and F2.10 with the head lights on. If there's NO POWER into both fuses the next spot upstream to test is multi-function switch #13 RD/BK wire.

Note, the multi-function switch can be rebuilt. Take TONS of pictures during dis-assembly.

Sometimes it's possible to restore function by exercising the hazard button.

An option is to get a replacement salvage yard unit and rebuild it. Then you are not risking your daily driver. Try Car-part.com for sample prices in your area.

1999-2004 MY fuse panel schedule:
https://www.allfordmustangs.com/for...r-swap-wont-fire-please-help.html#post2669271

Are the new ones aftermarket/non stock?