If your car just has one "lazy eye," there's a simple way to check whether the problem is with the solenoid, rather than the vacuum actuator. There's a small relay that tells the lights when to open and close. Reverse the solenoids to see if it switches which one is working (shown). If so, you can deduce that that solenoid needs to be replaced. If not, your next option is to pressure check the lines. Attach a vacuum gauge, plug it into the end of the hose (where it connects to the actuator), and plug the other end shut.