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Fuses are not really switches. They don't have zero resistance when good. Fuses are actually RESISTORS that heat up and melt at some pre-determined point. And since they are resistors they get HOT, especially the ones for the higher amperage fuses. More heat is need to "melt" the thicker link for the higher amperage. Smaller fuses seldom cause problems, but the ones for 15A and above do! Have you ever place your hand on a fusebox during the night with all the car lights on? They definitely get hot!
In addition you may have corrosion problems at the connection points of the fuses. They basically are a blade/spade type connector. Some computer connectors try for a gas-tight joint using gold plate. Cheaper solder (tin/lead) connectors are considered much less reliable. I used to clean printed circuit connectors with a rubber pencil eraser which would remove any corrosion/discoloration leaving it bright and shining.
There is a reason the fuses are INSIDE the 740 passenger compartment. You may have heard about the 240 Fuel Injection 25amp fuse near the battery and behind the radiator which causes so many 240 no start problems. Electricians use a anti-oxidant goo on connectors. Talk to someone with a 240 (like me) and they'll tell you about their small brass brush, pinching the fuse end connectors and applying this goo to the notoriously unreliable 240 fuse block.
In addition if you have a corroded joint with a large current trying to flow through it, there could be a potential fire hazard if you leave the circuit energized for a long time as you generated more heat (and corrosion as melted plastic, carbon, etc) at the defective joint. I have solved situations like this in the past with only my nose!
You could also troubleshoot this problem directly with a multimeter or even a simple 12v test light, but you would need an accurate wiring diagram. By chance have you installed higher wattage headlights?
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