Swapping alternators

I want to swap my old alternator which has wiring connections bolted to the screws. The alternator that i want to fit has a plug in for the wires. I have brought the plug but it has 3 terminals for wires where as my alternator only has the 2 the red and blue wires……. I have no problem re wiring the alternator but i need to know what the 3rd wire is and where it goes on the battery and what colour wire to buy……

( alternator i want to instal in the polished one)
 i hope this makes sense please see the pictures below 

Not sure about the wiring as electrics aren't my thing but the voltage regulator on the new alternator looks rather old, may be worth removing it and seeing how long the bushes are and give the contacts a good clean up.
There's the 2 slot head screws which hold the regulator on.

New one here but check it's the right one and check yours on either alternator before ordering one, lots on eBay etc.

https://www.ebay.co.uk/itm/175736458678?fits=Plat_Gen%3AMK+I%7CModel%3AGolf&epid=249096940&hash=item28eab58db6:g:6~MAAOSw3ZFftqlB&amdata=enc%3AAQAIAAAA8EydTLdBXfyFXuLGcE7g4JCdAoPAmIXxQ9PApjuyUwvcMumnDubU%2FdXzoXBY6nxXrRTOvd75qoJWE7qTrHmLmftOs1ydxkYxTOujc7kHr7w9JH5If0pQubvfrjPi3bptIuK59rJYwZH55s4Hnu2aPk7jxb29OI0gopgSXKYCseBYpfbxvT8NgQr2V23gV7hBZ7JhDJwLKG0OIHImqqfWm6DupvV9W1rw12tJ7ZAzU%2B5A00g0YuxURhEXbZpmmHewLq4%2Fbp6rTEUqePIPkGWSsRFk3MdJb4lgi8ALmqkOVZo9o195AYOV2T6MqbMCEmHwJg%3D%3D%7Ctkp%3ABk9SR9rNsdGjYg

The OEM with 2 post one big and one little are referred to as Bosch, where over here on this side of the pond we called the Plug in ones motorola,  The Motorola can in 2 versions one with the. old style regulator, and one with a squareish sealed flat unit.

The Plugs is for 2 wires Hot +12V, and the little one for the blue exciter circuit.  Both +12 usually ran back to the battery. Some ran from the Alternator to the starter then to the battery.