Typically the clicking is the solenoid trying to extend to make "solenoid slug" contact but not having enough energy/voltage to extend. Quickest test would be voltage on the inputs of the solenoid. If very low, check battery voltage during solenoid excitation (or do this first as its easy). Second test would be the voltage drop between batt + and + input on solenoid during excitation. Next do the same with batt - and solenoid -. These can tell which lead, if any, has a poor connection. Obviously if input voltage is good, replace solenoid or starter depending on parts supply and motivations. You can also bridge the output terminals with a screwdriver (old school VW) to verify the starter is good, but that is somewhat of a just for fun. This overlaps the advice from posts above....The sun is shining! Good luck Larrie!