cooling system in VW Jetta 95 4 cyl not working

I have a 1995 VW Jetta 4cyl with ac in which the cooling system is no longer working. I first noticed that the car was runing a little sluggish so I stopped and checked up under the hood and there appeared to be a leak from one of the hoses connected to the engine. I had the hose replaced but the car started to overheat. The mechanic thought that the thermostat was going bad so they replaced it but still overheating. Next, they replaced the water pump. Still the car was overheating. Then they noticed the fan was not coming on without the ac going. Checked the sensors and and they appear to be good. I have just one question. Is there a better way to diagnose these problems. Some type of tool that could have saved me some money for all the work these guys are going to charge me for that still haven't resolved my cooling system problem. Also, what do you think the problem could be. Thanks in advance for any help or suggestions anyone can provide to this post.

The fan control module under the coolant resivour with lots of wires coming out of it. That control the fan coming on/off. ALso the fan thermoswitch on the radiator driver side with 2 huge wires coming out of it, they fail alot also.

Common mechanical knowledge would have saved replacing the other parts, if the fans don't run it overheats. Pull the harness connector on the fan module & see if bluish corrosion is leaking out.