That remote switch is simply a DPDT, center-off, momentary contact switch wired to supply one polarity in one of the "on" positions, and the reverse polarity in the other "on" position. To test the solenoid, just hook battery power to its coil terminals. One polarity turns the disconnect on, the other polarity turns the disconnect off. Easier than doing that, just hook a meter to the coil terminals while somebody works the wall switch. That should tell you if it's the wall switch or disconnect solenoid that's not working. It looks to me like that fuse on the solenoid might be for the power feed to the wall switch.
