ac compressor help

[12vtim]

Im new to this site my name is Tim, I recently purchased a 96 318ti, the previous owner said ac system just needed a charge. I took it to school i am in automotive classes and pumpped down the system added 1.9lbs of freon and 2oz of pag oil so the system is charged. My problem is the compressor will only kick on if i run a wire from an alternate power source. When it is on it blows nice and cold.I checked for power at the wire that connects to the compressor from the fuse box and got nothing my question is the relay that is in the car is a mercedes relay does anyone know if this is usuable in place of the oem one? A0025421419 V23134-A52-X335 12V

Any feedback is greatly appreciated.

thanks,
tim