Blinkers work fine in park, Both blink when in gear - 2003 - 3.4L
 

First time 2003 Monte Carlo owner, and enjoying the car. I searched the forum posts a bit, and see it's semi-common, but was hoping for some confirmation seeing as nothing in the forum matches exactly.
Car starts - Blinkers Function Normally - No issues.
Shift into gear and I get issues:
-Rapid blinkers. Both dashboard signals blink. Proper indicator side is brighter, opposite side is very dim. Outside bulbs match up with the brightness on the dashboard with one at about half strength, and one very dim.
-Rear signals are blinking at full strength, and does not mimic the front bulbs with the opposite side not blinking dimly
-Hazards seem to work as you would expect them too, but I do feel like they are slightly dimmer than I would expect.
-Highbeam Out Message OR Security Message will appear in the Message Center.

It looks like someone replaced the Turn Signal Switch because the paint on it looks brand new and the car is 20 years old. I did clean the ground point I noticed by the driver side headlight. Fuse for signals appeared fine. Did not see a relay labeled for Signals.

I saw one post saying the ignition switch may be the culprit of different issues. Another mentioned the Headlight Switch possibly being of concern. I feel like it has nothing to do with the bulbs, seeing as they function 100% normally until the car is put into gear.

Thanks to anyone who took the time to read my post...Any advice would be great!

Might be because it is late and I am tired.... Unfortunately, I am not sure where to start with this issue. Your symptoms seem very odd. Do you know if there is any aftermarket changes to the electrical? Maybe someone spliced into the side that is dim and there is an electrical issue with what was done. If you do know of any changes, I would explore how to remove them.

Yes, I find it super weird it only happens when the car is in gear. I'm mechanically inclined, but this one, I wasn't sure where to go with it. It does have an after market radio in it with a touch screen. The hazard switch is loose in the opening it's supposed to stick through, but it seems to function normally. I drove it in the dark tonight, and I'd say that if I signal left, the Left Signal interior light on the dashboard appears to be blinking bright, but the outside left turn signal bulb is dim. It's awfully weird. Considering a lot of other people said ground issues could be the cause, I figure I'll start there tomorrow. But idk why the ground would work in park, and not in Reverse/Drive or any gear. Thanks for the message

I don't think the issue is the hazard/flasher module. That is a pretty simple device (it's behavior does not change based on gear). Might be worth checking if any other lights come on when you put the car in gear. Such as, does this have factory fog lights? If someone added fog lights, how did they wire them in? Possibly tapping into the turn signals (which would be weird). Like I said, check for anything aftermarket.

Something that comes to mind is, the DRLs turn on when you put it in gear. Maybe that could be a lead?