Remove the engine cover and disconnect the battery.
View the wires leading to motor control unit. If the wires are corroded, scrub with a wire brush or replace.
Replace the solenoid. It should be held on with a few bolts and possibly a metal bracket held on by a screw. There will be at least two wires on the back of the solenoid; they must be removed and inspected and replaced, if necessary.
Reconnect the battery and test. If there was a faulty solenoid or corroded wire causing the problem, the motor control should work.