Side Marker LED Swap

Those look like 5050s to me. Just measure the white part of the LED package with a caliper. It should be 5mm square.

You'll want 12V versions, of the type that is a single color only (there are types that have separate RGB signals, and types that are controlled with a serial data stream.

Hmmmm...

There was no real data on the second one you linked to. The first and third ones you linked to clearly says they're for automotive use. But they're only 3V diodes. I can't tell if there's anything on the back side of that PCB, but on the front I just see resistors and a diode. And while it's possible the resistors are arranged as a voltage divider to provide 3 or 4V to the LEDs, I don't see why GM would have done it that way when they could have just used 12V LEDs.

So what I'm saying is, it's possible those LEDs you bought will burn out the moment you power them up.

Do you own a Digital Multimeter?

The meter's Diode Check function expects the diode to not be in a circuit with other components, so I'm not certain the measurement tells you anything more than the LED lighting up tells you - when forward biased the diode lights up. Though, indirectly, that's probably enough, it suggests the diode is a ~3V LED.

What the meter is telling you is what the voltage drop across the diode is. Once the voltage across the diode is equal to the voltage drop, the voltage drop will be the same no matter what higher voltage is applied to the diode. We don't know what the meter's voltage was, because the diode test is a current-based test, but it's likely the meter increases current until the voltage drop stops increasing, so the voltage it was at when that point was reached was the voltage actually displayed, 1.79V. If it was a 12V LED it wouldn't have lit at all at that voltage...

Perfect, so far it's looking like the LED's I ordered may likely work. I expedited some over SMD's and they are stated to arrive this weekend.

@RocketSled Thank you so much for your help and guidance during this process.

I'm hopeful we will get them to work.

The guy did it this way because he lacked the skills to do it right. The LEDs can be had on small PCBs, which it appears he bought instead of discrete components.

What you did should have worked.

Can you check for codes to see if the BCM is complaining about the side marker lights?

My guess is, the LEDs you bought are drawing too much current so the BCM shuts them off (think "circuit breaker" behavior). Do they maybe flicker, come on briefly before turning off?

No flicker and no codes.

Explain with more detail how you tested them, when you got them to light up? And while I don't expect to see anything, give me a closeup of one of the completed boards?