When you see a direct battery connection like this, it usually means the relay is internally fused. Later relays had a plug in fuse which could be changed. But with the early ones, if the fuse went, the relay was toast.

Since the main fan comes on when you short the sensor, your climate control head unit is ok, which is good news. This probably means the entire high speed circuit is fine. But high speed will only come on when the engine is very hot...probably around 105C. The low speed is triggered by the pressure switch on the receiver/dryer. Most likely problem is a fried resistor, as has pmckechnie pointed out...test it for continuity. If the resistor is ok, replace the relay and you should be fine.

I'm adding some photos...the first is an internally fused relay, as found in my '87 190DT. Notice the schematic on the top of the relay shows a fuse. These are really awful...you can't tell if the fuse is good by looking, and can't replace the fuse. When the fuse blows, it actually starts the plastic burning, but there isn't enough air in the case to sustain a fire, unless the case is compromised. The other photo is the new design, with a removable fuse in the top. The new one is plug compatible with the old one, and should be substituted before you have a problem.

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