why does my fan that cools the motor only turn on when i turn the a/c on?

Not knowing all the facts I would say the fan comes on with the A/C because the system is sensing the need for more "air flow cooling" due to the "anticipated" drag the A/C system will put on the running engine, in assumption that the driver will push there foot down on the gas pedal more once the A/C compressor "kick's in" and come's on.

do know why my fan is not coming on when its supposed to? I put a new thermo sensor switch in yesterday..

Take your fingers and spin the blades of your fan. Do they spin freely? Or do they bind up and make noise? If they bind or don't spin at all, your fan motor is shot.

If I remember correctly the Volkswagen has an electric fan mounted on the radiator ? You said the "thermo sensor switch" was replaced, So how many miles are on the thermostat (in the engine) that is in the car now ? If you don't remember or don't know, I would change that before going any further.

The A/C requires airflow over the condenser in front of the radiator to transfer heat from the circulating refrigerant. There's a relay that engages the fan when the A/C turns on. If you run your engine up to full operating temperature and the gauge needle goes past the midway point, the thermoswitch in the radiator should also turn on the fan. If it doesn't, you may have defective switch. You can test it in a small pot of boiling water with multimeter leads clipped to bot tabs to read resistance. If the switch resistance goes to zero, it's working. In that case, you may need to chase down the wires or the fuse.