I have a 1981 Chevy Luv with the Isuzu Diesel and it won't start. When I turn the key I get 1 click. With the headlights on I still get the click the lights dim but only a little and as soon as I let off the key they go right back to being bright. The battery is charged and the terminals and post are clean and tight. Is this a loose connection further down or is it the starter or solenoid?