On the Window Switch Contacts (in my case burned some) I used a small Pocket knife to scrape off the burned stuff.

For male Electrical Plugs I have used 220 or finer Wet&Dry Sand Paper.
For Femal Sockets that a Blade fits into like the Alternator Connector I have folded the Wet&Dry Sand Paper and slide it in and out to clean up the socket.

For some reason round sockets do not seem to suffer as many problems as the blade type do.

What you uses is going to depend on how bad of condition what needs cleaning is.

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84 300D, 82 Volvo 244Gl Diesel