Do you have to take the dash apart replace speedometer on a 1991 Mustang?

Generally, yes, you will have to at least partially disassemble the dashboard to replace the speedometer in a 1991 Mustang. The speedometer is typically located behind the instrument cluster, which is itself recessed into the dashboard. You'll need to access it from behind, meaning removing or significantly disassembling parts of the dash. The exact level of disassembly will depend on the specific model and your comfort level. Some people manage with less disassembly than others. However, complete removal of the instrument cluster is almost always necessary.