Speedometer stopped working

My coworker had a buddy install his new sound system, complete with bass, amps, etc. When he went to go drive it, he realized the speedometer wasn't working anymore. The rest of the gauges are working fine, the plug in connectors are all intact. But a hack job was done on the radion wiring. Could there be a common wire between the two, say as part of an anti-theft system? It's a 2000 Blazer.

well, my quick guess would be that somebody cut a wrong wire in the dash.

maybe hooked the power up on a wrong line and blew a fuse, or disconnected a ground