Passenger turn & side marker lights trouble

On my 76 Burb, I can't seem to track down what's causing my passenger side turn and side marker lights to not work correctly. Here's how they are functioning right now:
* Turn on hazards = all lights work but passenger turn + side marker lights which don't turn on
* Engage right turn on stalk = passenger turn + side marker lights don't turn on but rear right turn lights work
* Engage left turn on stalk = works as normal
* Turn on headlights = all lights turn on including passenger turn + side marker lights (right turn on stalk still doesn't engage blinking but lights remain on)

I did the following to try to address it:
* Replaced bulbs -- side marker light was out but now works
* Cleaned out the only grounds I could find on the passenger side of the engine bay which were near the front grill area -- one of the wires did seem to go to the side marker light
* Checked the rear light wiring underneath the rear bumper, only saw one ground and it looked clean and tight

Only other related note I can think of is that the turn signal stalk doesn't stay in the up position when I use it to signal a right turn. It flops down to center right away. When signaling a left turn, it seems to engage a bit stiffer but can't tell since gravity may also just be keeping it down.

Any other ideas I can follow up on to get these lights working?

From what you describe, the most likely cause is the turn signal switch. You can check that switch using the connector to the switch at the bottom of the steering column. There are many more places it can fail so you need a wiring diagram and a test light or meter.

Dark Blue-15 wire. If I remember correctly (not home currently). Comes from turn signal switch and runs thru bulkhead to right front turn signal. Could be bad turn signal switch or something with DK BLU- 15 wire or it’s connections

Does the dash right turn indicator work? If it does, problem is probably after bulkhead. If it doesn’t, problem is probably before bulkhead(such as turn signal switch).

Yep- DK BLUE-15

side note- always double check bulbs are good and you are using the correct bulbs. Front turn signal is a double filament bulb.

Your 1976 Suburban's passenger-side turn and side marker light issue is likely due to a problem in the wiring, grounding, or the turn signal switch itself.