i'm pretty sure you just pull the speedo cable out of the tranny (it is held in by one bolt.) and there should be a c clip type thing holding it in place. I've done this atleast 5 times on autos, 5 speed should be similar.
- '90 Mustang LX-Sold "Debbie"
- '89 Mustang GT- Deceased "Lucy"
- '91 Mustang GT-70,000 mi "The Unicorn"