In that case, if it has the adjustable center cable, you might need to tighten it up a bit. It's a common issue that fox mustangs with SN95 rear brakes have weak parking brakes unless you tighten them to the point of just before the rears drag
Basically put the car on 4 jackstands in nuetral and have someone turn the rear wheels while you tighten the center adjustable cable until they start to drag. Then back it off to the point where 2-3 clicks up is all that's needed to have a good hold that should hold on steep inclines without being in gear.
Of course, this is assuming whoever did the install use an adjustable cable. if they used Sn95 cables, it's possible the cables are stretched.
The e-brake does have a self-tightening mechanism. Usually, with the adjustable center cable, it's welded to defeat it and the adjustment is through the center cable. If it isn't defeated, it usually rachets the cable up too tight stretching it.
Basically...there are a few things to check out