Not sure about a Fiesta and also what would have been the system in 1992.
Caravans have a pump which goes into a water container outside. The pump has a hose which plugs into an inlet in the side or under the caravan. This also has electrical connections plugging in.
Inside the van the hose goes to the tap and possibly also a water heater, if fitted.
In 1992, the pump would be switched by a microswitch on the tap which clicks when the tap is turned on and off. It may be a hand or foot operated switch instead though.
The power for the pump would be 12 volt fed from an onboard leisure battery, or in the case of a folding camper, maybe a couple of big dry batteries like would be fitted in a large hand lamp.
In a camper, the pump hose may not need to connect via a plug in inlet. It could simply pass through a gap in the canvas to the back of the kitchen unit.
You will probably find a drain hose from the sink either passing down through the floor or to a connection point outside. This will go into a waste water container.
You could simply buy a water container with a tap on it which could sit inside by the sink. I have a 15 litre one of these which lasts a fair time. Need to unscrew the lid slightly though otherwise if air cannot get in the water cannot come out the tap very fast.
We had a Pennine Aztec when we first started and the Fiesta had not started production in those days.
We were so delighted when I purchased a faucet to fit on the sink and connected it to an outside water bottle. Using a submersible pump connecte by a press switch on the side of the kitchen unit to supply power to the pump from a car battery. Gosh, it was the last word in luxury camping.
We had a 1999 model and it had a balloon type switch on the floor below the sink to pump the water, (cold only).
Has the camper got, firstly a battery fitted and secondly this type of switch on the floor?
------------- I came into this world with nothing and I've still got most of it left.