1990 B250 A/C and power windows stopped working simultaneously

It sounds like you replaced the tumble assembly and not the actual ignition switch. There is a rod that goes between the tumbler and the switch. When you turn the key, it moves the switch into different positions. It could be as simple as an adjustment issue of the switch itself, or it could be bad. The switch is located on the steering column. I have attached a pic for you to look at. Let me know if it helps.

The lock cylinder doesn't have anything to do with this. In fact, if you look at your old one, it doesn't even have any wires on it. It's the ignition switch you need to look at. That is halfway down the steering column. You must check that it is not loose because loosening the two mounting bolts is how you adjust it. When you observed things worked intermittently when you turned the lock cylinder a little, you just proved the fuses and fuse link wires were okay because those are never intermittent. When one burns open, it stays that way.

Most likely the ignition switch is not loose. The more likely suspect is a pair of contacts inside it are arced and burned and the switch must be replaced. Unplug the connector and check if two terminals are black and the plastic housing around them is melted. To repair that, once the switch is replaced, those burned terminals have to be replaced too. If you don't replace them, they'll make a less-than-perfect connection which results in heat buildup, and the same thing will happen again shortly.

Those two terminals can be cut out of the connector body, then they are replaced and plugged into the switch individually after the connector is plugged in. You'll need to replace about four inches of those wires too because the heat has hardened them and solder won't adhere. Splice in four inches of new wire of the same diameter, solder those splices, then seal them with heat-shrink tubing. Never use electrical tape in a car because it will unravel into a gooey mess on a hot day. You can use crimp terminals to replace the two that were burned, but solder them to the wires too for a better connection.

This problem happens most often to people who use the heater fan on the higher speeds a lot because of the very high current flow. Also turn that switch to the lowest setting before the ignition switch is turned on or off. Turning the ignition switch on and off when the fan is set to the highest speed causes a big arc on those contacts and leads to this problem. It's always the same two terminals that overheat. It's the pair for the heater fan, power windows, and other things that are switched on, like the radio and wipers.