In my 1995 Buick Century 3.1 the power locks, headlights, tail and brake lights, interior lights, radio/clock memory have all stopped working. The power windows still work. I've replaced fuses, but don't know where to start after that. I'm thinking it may be a relay but don't know what one. How do I know what relay to replace?

The dash and tail lights are on two different circuits, and the brake and turn signals are on different circuits, so at least 4 circuits total.
I would start at the headlight switch and check for power on the first two issues.

Hope this helps your out. I know the diagram and locator is a bit blurry but hopefully it gets what you need.
Drivers kick panel:
Front wiper 20A - Front wiper system Cigar 15A- Cigarette lighter Audio 10A- Audio, remote control mirror Air Bag 10A -Air bag diagnostics Stop lights- 15A -Rear brake light, high mounted stop light ECU B+ 10A - Shift lock actuator, transaxle controle module, radio, engine control module Room 10A - Interior light, clock, ignition key reminder, rear cargo area light switch ABS 10A- Anti lock brake system control unit ECU IG1 10A - Transaxle control module, shift lock actuator, ABS control unit, DRL unit Meter 10A - Instrument panel, back-up switch, electronics time control module, inhibit switch Turn signal 10A- Hazard switch ,front and rear turn lights, turn indicator IGN1 20A - Ignition switch ABS 30A- ABS pump motor TNS 30A- Tail light, license plate light COOL/FAN 30A- Cooling fan Cond/Fan 20A- A/C condenser fan Starter 15A- Start signal , Fuel pump Blower 25A- Blower Motor SR/ACC 10A - Ignition switch Hazard 15A - Hazard and turn signal light D/Lock 25A- Door locks ABS 30A - Anti lock brake system control P/W RH 20A- Power window motor right hand P/W LH 20A- Power window motor left hand IGN2 25A- Ignition Switch
Engine compartment :
Head 25A- headlights IG 2 30A- Ignition switch IG coil 15A -Ignition coil, condenser,generator IC regulator, diagnostic connector. O2 down 10A- Rear O2 sensor O2 up 10A- Front O2 sensor Fuel pump 10A -Fuel pump unit Injector 10A -Fuel injector ,camshaft positioning sensor, purge solenoid valve, EVAP, IAC actuator, Mass Airflow sensor, EGR stepper motor. fuel pump relay coil, vehicle speed sensor. A/Con 10A - A/C compressor controls, cooling fan BTN 30A- Brake light , high mounted stop light. Tail light LH 10A- License plate light, tail light , rear combo light, instrument panel illumination Tail light RH 10A-License plate light, tail light , rear combo light, instrument panel illumination Head low 15A- Headlight Head Hi 15A-Headlight Horn 15A- Horn

Inside kick panel

Your brake light switch was sticking and now the fuse box is shorted. between those 3 systems. it happened to me in a 90 ford it was wipers brakes and radio. I took fuse box down and taped wires, and replaced brake light switch and it fixed it

Be sure that the battery cables are clean with absolutely no corrosin on them.[Baking soda and water mix works really well.]Check the fusible link wires at the starter. You can pull on each one , if it stretches, it is burnt in two.

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Your interior lights fuse is powered by the tail/park lamp fuse, that way you know when your parking lamps are not working. Your interior lights probably went at the same time, but you noticed them first. Most likely, you have a blown park/tail lamp fuse. Check the fuse first, replace it if it's open. If it blows again, you have a short in the park lamp circuit.

BTN stands for Body/Tail/iNterior. It protects the HAZARD, ANTENNA, ROOM, STOP, ILLUM and ABS fuses in the cabin fuse box, which in turn protect the hazard lights, antenna, ECU memory, brake lights, tail lights, ABS, power locks, clock, horn, illumination, stereo, HVAC, interior lights and alarm. __________________

two places left drivers side under the dash are on the side of the dash by the door for the interior, some cars have them on the right side of the dash also. There is also another fuse box under the hood. If you have your owners manual this is all listed in it, and the exact fuse location for your breaklights, I think your will be on the drivers side interior Hope this helps you! Merry Christmas and God Bless!

It might be a electrical wire shorting or bad.

I had solved a majority of my problems by getting my electrical system looked at (turned out a brake wire was bad, and several needed to be replaced.)