78 Series Troopy Speedometer Issue

[Jatmeakins]

Hi new to the club and looking for some help. My daughters 2004 78 series troop carrier has a fault in the instrument cluster that I am finding difficult to diagnose any advice appreciated. Speedometer works fine, however when the sub tank is switched on the Speedo stops working. Also noticed when I remove the globe from the cluster for the sub tank light the speedometer stops working.

thanks Kevin