Ford 2003 Mustang Workshop Manual page 2825

3. NOTE: If a new rear output shaft bearing was installed or a new rear countershaft bearing is
being used, install new bearing cups.
Install the rear countershaft bearing cup and the rear output shaft bearing cup.
Lubricate the bearing cups and the shift rail bearing with petroleum jelly.
4. Install the rear countershaft bearing.
5. Install the reverse idler gear bearings and spacer on the shaft.