Yes you need a 75a relay. I forget where I bought mine, it was years ago when I got it. Try doing a search for bosch 75a relays online and see if you can find places that sell them.
I forgot the link or the name but I was recomended to buy this controler from someone on here, it works very good, and since then I have never had a problem.
If you do buy a relay, I think GM has a thermostat that will send a signal to your realy so you wont have to use a switch