Sounds like your clutch is dragging. It's either the clutch or the auto adjust is worn out. I would try one of those adjustable clutch cable setups first.
Same thing happens to me. Trying the clearing technique doesn't help, sometimes it just grinds and is hard to get into reverse. So I just park in the lane in parking lots and that fixes that... Now I just get parking tickets
Before hitting reverse : 1st neutral , 2nd 5th gear( or any gear but 5th is a direct path to get to reverse) , 3rd reverse , it's just a habit to take and trust me it will never grind.
Newer Stangs don't do that with the TR3650 , they have something to stop the gears from spinning when you hit reverse but not the T5 and T45 !
Its an issues with the T45 transmission, which your car has. Its got no synchros for reverse, hence the grinding. To work around it, put it into first, then reverse and that should do the trick.
Before hitting reverse : 1st neutral , 2nd 5th gear( or any gear but 5th is a direct path to get to reverse) , 3rd reverse , it's just a habit to take and trust me it will never grind.
Newer Stangs don't do that with the TR3650 , they have something to stop the gears from spinning when you hit reverse but not the T5 and T45 !