I also have an eheim and enermax and was going to set mine up like your tyring to do yours, but I opted instead to just solder the pump to the leads comming off of the IEC connector (power plug) so that the pump is always on as long as the powerstrip is turned on or the computer is plugged in. This was much easier for me and the eheim pumps are high quality and have been designed to run 24/7 in aquarium setups for ever. Also, I have read somewhere that the turning the pumps on and off puts more stress on the drive than just leaving it on. Like a light bulb. This will also reduce the strain and probably extend the life of the power supply. If you really want a switch, I would suggest putting it on the pumps AC line and mount the switch on your case, which could look really cool with a nice LED. And if your worried about pump noise from leaving it on all the time, dont be... once all the air is out of your system it will make no noise at all (It does make a little noise if you put your ear up to it, but realisticaly I cant tell that its on), I also put down some sound/vibration absorbant material on the inside of my case which also helps to eliminate the vibration of the pump, which seemed to me to be the only big source of noise I could really pin down and get rid of.
good luck!
