No brake lights, rest of lights are fine

you have the wrong light bulb

Check the wiring in ur running light, somebody might have made mistake repairing it, CHECK also the grounding!

dirty corroded ground either at wire to body or bulb socket where bulb is inserted look for dirt and corrosion.If available use a noid light and a brick to apply brakes to check for power at bulb socket

Yes switch is the problem. Volvo claim they sell very few switches, probably because people bought new cars instead. Problem can be associated with failed ignition coils (2 of) but one does not cause the other but very confusing. Engine fails intermittently or conks out.
Reset battery earth connection on chassis bad design here. Clean off paint and thread also bolt coating. Even better, use brass bolt but do not overtighten. Volvo will try to get you to replace alternator which is not necessary.
Failing battery will light up all the warning lights but will not isolate the battery so if it shorts car will die and not run on alternator. If this happens disconnect battery until you get to garage.
Now you know you can buy an old S40 with confidence great car and very cheap for what you get.

u need a brake light unit

google 1991 volvo brake light
There several stores that have lamps.

Solder to the circuit board, just in case they interact with other circuits.

had the same prob..check the the trunk where the bulbs are in the light bezel there's these flat steel contacts where the bulb housing is making contact with - fiddle with those a bit and you'll find that they'll come on when you move em around a bit. They're made pretty badly.

more likely to be the brakeswitch

Is there a switch on the left side of the dash that has a red lens and a wavy line across it? If so, you have a rear fog light switch and it has been activated, causing your brake lights to come on each time you turn your headlights on. Flip the switch and the rear lights should display only the parking lights with the headlights on.