How hard is a block heater installation on a 2009 Mazda 6 2.5?

It is not hard. You can see the freeze plug on the lower driver side rear of the block, remove it with a 9/16" allen key socket and catch the coolant in a drain pan. I installed an oem block heater for a 2010 Ford Fusion and almost of the fasteners lined up with something. You need a big socket (1 3/8" I think) to install it, route the wire to the front passenger side of the engine compartment. Drive the car a few miles and top off the coolant expansion tank reservoir ( you might have to do this a few times) and your done.