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Nissan PrairieJoy: Found this unit inside the door panel, with its multiplug connector. When undone, all parts of the car still seem to work perfectly, so I wonder what it is for?
It's a Nissan component, marked B-KEY2, along with a bunch of identifying numbers. It seems to have an aerial attached, and has an internal buzzer.

This sounds like a receiver for the remote central locking, or even for keyless entry (proximity key). You don't say what year the vehicle is, but some Nissan models, particularly for the domestic Japanese market, had keyless entry from quite early on.
When you say everything still works, have you checked and identified whether your vehicle has these options?

Thanks. 97 model. The car only came with 2 ordinary keys. Do you know if there is any way to access the 'keyless' option?

Being a '97 I think it would be too old for keyless entry. It is most likely to be a receiver for a remote key. If you don't have the remote keys, there's probably not a great deal you can do that would be cost-effective.
I suspect the cheapest option for you to get remote locking/unlocking would be to go with an after-market option, available from most car audio or alarm outlets, but I'd imaging you'd be looking at a minimum of around $250 to $300 for a reasonable quality system.