1999 Audi A6 Cooling fan not coming on

A few months back I bought a 1999 Audi A6 from a local used car dealership. Once I got the car home I noticed there were a few cracks in the coolant reservoir and that the coolant was the wrong color. I drained the coolant from the car and flushed the system with water from the hose (It wasn't until after I had done this I was told that I should have used distilled water). Once I was satisfied that all the old coolant and potential debris had been flushed, I filled the coolant system with new 50/50 pink (R12? I think its called) coolant and water and bled the coolant system to remove any air pockets.

A few days later, while sitting in the drive thru, the car got hot enough for the alarm to come on. I pulled to a parking spot and popped the hood. I found coolant leaking from one of the connectors on the reservoir so that afternoon I replaced it.

Ever since then I have had to turn the heater to defrost to make the coolant fan come on in order to keep the engine at an appropriate temp. This may have been an issue before though I may not have noticed it (because the weather was cooler).

I have read through some forums and found a few answers but nothing concrete. One of the solutions that I have heard was to replace the fan control module. This doesn't seem right because the fan will still turn on as long as I have the heater on. I am leaning towards a coolant temp sensor, this to me makes the most sense. I am definitely no expert though so any help would be greatly appreciated.

Side Note: I have noticed that the engine temp stays a bit lower when I am at lower speeds. A lot of the posts I have seen regarding this kind of issue, the poster experiences the opposite.