temperature gauge not working

If the fans still come on when they're supposed to, could the thermostat still be defective?

Probably not unless they use a separate thermal sensor aka "otter switch" built in to the rads like Jaguar and Jensen used to do. I haven't been able to find a circuit diagramme for the car, aka "Sunny" then "Almeira" in the UK.

Oooo! Grounding "sender" wires! Nah - cool for quality - I mean, older cars! Fans switches are separate to temp senders (usually). I think the switches tend to be called "coolant sensors". (To me that is a fluid detector). Often one or 2 wires. The resistive gauge senders etc temp sensors etc. Usually single wire. I'm off to a wreckers yesterd... today so I'll check some '94 Nissans. Rats - I know the N13 Nissans (or rather, their Delco EMS and 70A alternators - my first upgrade from 23A with external mech alternators). FYI - though gauges may operate off ~8V (eg, fuel & temp; usually not oil pressure), older cars use pulsing bi-metal voltage regulators, hence either on or +12V but averages out to ~8V. Pulsing may not start until the (thermal) gauges have hit their correct deflection.    Just a potential trap for those expecting to find or check 8V with raw DMMs etc. (And there is never any need to worry about reverse polarity for those gauges - they are non-polar. But avoid raw 12V into fuel tank senders - just in case of oxygen!)

Yellow wire? Single. Not 2-wire. 1993 U13 is a Bluebird down here, but no B13 found. No radiators in the few samples, but saw a similar with temp sender on gearbox end of engine (E-W engine; aka our & yUK's nearside, stateside's offside) and 2-wire fan switch nearby; on end of head, down a bit. But yellow is very likely to be the wire color.