Since you were able to get idiot lights and gauges you had the plug aligned correctly at one point.

The preglow system and the seat belt warning buzzer problem is likely related to the ignition switch replacement. The two work together. For instance, in my 240D, the seat belt buzzer will go off for the duration of the glow cycle if the belt isn't buckled.

If you replaced the ignition switch with the battery connected you may have shorted a pin and blown a fuse.

Either way I'd replace that connector - it's in bad shape. The cluster has some diodes in it that could be damaged and gauges can be destroyed. You can replace the shell without splicing by separating the halves. When you do be sure to press the harness into the connector. Otherwise once the connector is separated it will fall right off leaving you looking at wires and pins. While holding the connector on take careful note of the pin locations. Make a diagram ahead of time and fill in the colors. A helper is recommended. If you have a problem I can look at one of mine and give you the pinout.

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1969 220D 5 Speed (OM616)
1983 240D 4 Speed
1985 300D Auto 376K
1985 300D Auto 275K