On a 72, the blower should bolt to a bracket that bolts to the top of the cooling fan shroud.
The wires come off of the relay board. I know this is for a 73, not a 72, but the board itself is unchanged.
http://www.pelicanparts.com/914/parts/Electrical/914_electric_73E.jpg
On the 72, there is a green wire in the connector at the right-rear of the board. That powers the blower. There is a separate ground wire that winds up attaching to the ground point under the relay board.
--DD