ebrakes not holding

Check BOTH sides for slid pin galling to the bracket. Common Mustang problem. The slid pins MUST be able to move freely for the brakes to adjust and hold correctly.

If one caliper is replaced, the other on the rear should be replaced as well. These things should be replaced in pairs. The shop should have told you that in advance. Your problem is one of 2 things. 1 - as I stated, they should be replaced in pairs, and the problem could be that the drivers side calipers piston is so worn that it moves too easily allowing the drivers side to move before the passenger side. or 2. the slide pin on the passenger side is sticking due to corrosion and/or lack of caliper grease. Clean corrosion off pin and apply grease to pin then reassemble.

One other thing I forgot to mention. There could be air in the brake line on the passenger side, not allowing it to function properly. It's possible the lines weren't bled properly.

The drivers side caliper could be sticking IN a little but not retracting all the way. Thats a slight possibility.