This is a Trico Electro-Vac. When energized with the "vacuum switch" (see picture) it will detect loss of vacuum, and the electric motor powers a vacuum pump to keep your wipers moving. It does not run constantly, only when loss of vacuum occurs during wide open throttle conditions. It is installed in-line with the vacuum hose from intake manifold to wiper motor.
I found this on the firewall of my COE during restoration. It works perfectly and is capable of pulling 11-12 inches of vacuum all by itself. My wipers function with the engine off, ignition on and vacuum switch on.
These are not factory options, just Trico accessories. Instead of designing drop-in electric motors to alleviate the dreaded loss of wipers when going uphill, they designed this so they could keep selling vacuum motors. They are not common and most are 6 volt. I saw a NOS 12 volt Electro-Vac sell for $1200 on Ebay. The switches are even more rare.
