No crank on v6 (atq) passat wagon 4motion

Virtually every starter have three terminals on the starter solenoid; S, B and unmarked. The unmarked terminal may be a bare braided wire. Ignore it. The B terminal may have two wires; large red battery cable and a smaller wire that runs to the alternator for power. This small wire is a fusible link; its small gauge wire is sized to burn out if overloaded to protect the battery. The small single wire on the starter solenoid is the S or start signal from the ignition switch to powerup the solenoid. The solenoid has internal contacts that connect battery power to the starter motor thru the braided wire. A simple test for the starter is to short the B terminal to the S terminal to power up the starter without using the starting circuit. A short length of 10 gauge wire will suffice. There will be some sparks as the solenoid draws around 5-10 amps. Be sure you feel comfortable under the engine with parking brake engaged and transmission in Park (neutral in manuals). Leave ignition OFF as all you're doing is testing for starter operation. If the starter doesn't power up, be sure battery cables are clean and free of corrosion, including battery negative; battery to chassis and chassis to engine block. Its presumed this is the original starter with 20 years of normal use as starter brushes eventually wear out. My guess is a worn out starter. Sometimes, whacking the starter body with a piece of 2x4 may allow another start as worn out brushes make contact to the commutator on the armature (shaft assembly). Some are rebuildable with ordinary skills but a source for parts is needed; bearing, brushes, etc.