No power supply

Hi all, please I need help with my agu golf4 airflow. When I check power on my 4 pin plug none of them has power, but if I start the engine I can read 13 volts but on that same wire if I turn on the engine it shows me it is a negative checking via negative continuity with a multimeter. So my problem is that I don't get a signal from the air flow. I've them I time but still facing same problem. Is possible that positive wire has a fuse? Can someone help with a diagram? Thanks in advance.