240 1987 Sedan brake light is out, how to wire the 3 prong socket?

I got pulled over earlier this week due to my driver's side brake light going out. I figured I might as well try to hardwire the brake light socket alone as the others still work fine. However, the 5W/21W sockets for brake lights have 3 prongs instead of 2 like the others. How do I know which prong is the running light when my headlights are on and which is for the brake? I'm assuming all the downward-facing prongs are grounds. I also have the fog lights if that matters with how things are wired.

https://web.archive.org/web/2018041.../volvo/240 Wiring Diagrams/Volvo 240 1987.pdf

Look at page 17, lower right corner. Wire colors are different depending on which side you're trying to work on.

For driver's side:

Left tail light circuit is RED wire (confirm by checking for 12V with multimeter when key is on and lights are on). You can then follow the copper trace on the PCB to see which contact on the malfunctioning socket it goes to.

Left brake light circuit is yellow/gray wire. (Check for 12V with meter when key is on and brake pedal depressed or brake light switch jumpered). Likewise you can follow the copper trace to the appropriate contact.

The ground is one of two black wires. It should be pretty obvious as it'll have traces going to all the light positions.

I'd definitely make sure you have power to all the appropriate wires. Always possible the bulb failure relay (if still present) has failed and power isn't getting to your brake light.

Why do you wish to hardwire? What's wrong with the connection as per original?

I do not recommend hardwiring because it makes like difficult if you ever break a taillight and need to replace it

I've found that the main reason a light fails to energize is a break in the copper trace serving that light.

Usually the break is caused by the metal prods on the socket gouging it.

To diagnose, energize the target socket and use a test light to trace the flow of electricity to the area of the break, then cover / repair the break using some form of liquid metal; I've used a bit of the fluid that comes with the kits used to repair breaks in rear window heating strips.

I replaced the circuit board and used dielectric grease on the copper contacts and socket prongs so they slide on easier, lowering the chances of the prongs scraping/gouging the copper. Maybe it'll also help prevent early corrosion? I noticed that the brake light copper on the old board looked oxidized, the markings from the prongs were nearly black in color. As of right now, the brake light is working again and the light failure bulb is off.