The 96-04 model year Mustangs do not have a fuse panel "under" the seat. Is it possible this is a "modification" to support an Urber sound system?
To have multiple electrical problems with the cluster screams "gounding". Check the cluster ground behind the center console. It is shared with multiple devices including the radio.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.