Just lost all 4 headlights on 91 RS. Tail lights and dash lights still working.
Thinking switch or relay, yes/no?

Where's the relay located?

 

Your car has no relay and no fuse for the head lamps. The parking lamp/ head lamp pull switch and the low beam/ high beam (DIMMER) switch on the steering column are responsible for allowing 12V to the head lamps.

 

Check "fusable link A" , which is said by the schematic to be "rust" colored . This fusable link feeds the headlight portion only of the headlight switch , and if bad would put your headlights out while still allowing your instruments and tail lights to function since their portion of the headlight switch gets power from a different source (the 20A fuse marked "tail" in the fusebox) . The solid red wire to the headlight switch is fed from this fusable link , so a 12V automotive test light will tell you if the red wire has power . If yes , turn the switch on and check the yellow wire , this leads from the switch and goes to the High beam/low beam switch . If the yellow does have power , then next check the tan (low beam) and the light green (high beam) from the dimmer switch for power in either position . If , in the odd case you DO have power on either the high and low beam wires , next up could be any connector between the high beam switch and the wiring itself , or maybe you lost the ground side to all 4 lights all at once , or , (crazy unlikely) maybe a monster voltage spike took em all out ? Also , just as an FYI , there is a circuit breaker built into the headlight switch itself and is part of the switch and not separately replaceable . This is a self resetting breaker and if your lights are drawing too much power it will turn the lights off and on , the speed of the blinking depending on the severity of the overload . Drawing quite a bit over ? they will blink fast . Only somewhat overloaded ? they will blink slower .

The above wiring info taken from austinthirdgen 1991 body wiring diagrams .

 

Thanks guys that should help chase it down.

 

What if we change the fusible links and still have no power to headlamps what else could it be

 

If an unprotected direct power-source is connected to the the Headlamp-Switch...
and the the Headlamps still do not operate; it is possible that the Headlamp-Switch itself is inoperable.
If disassembled; the Headlamp-Switch contains its own built-in Circuit-Breaker... it can fail in the OPEN position.
Also the "Dimmer-Switch" (the actual device that switches between Low-Beam and High-Beam Lights; not the Multi-Function Lever) can also be inoperable.

A Digital Multi-Meter is an invaluable tool in this situation, to test for continuity through these Switches/ Components.

I would however still test all the Head-Lamp circuits continuity as well.

 

Thank you OrangeBird!

I slept on this one...
I appreciate you stepping in as you always contribute solid information!