Dim brake light LEDs (driver-side)

Hey Prius folks,

I recently noticed my driver-side brake light LEDs are much dimmer than the other side and am trying to figure out the root cause. All 6 of the LED grids in the taillight assembly are lit but dim and are also flickering between maybe 25-50% brightness; I guess this means either insufficient current or voltage?

All of the 3 other bulbs in the assembly (tail, turn, reverse) work just fine and at full brightness. From reading around, it sounds like there are a few possibilities:

1) A bad ground connection - I checked the bolt underneath the trunk latch and it's tight and shows no corrosion. It looks like this ground is for the whole driver side assembly so since all other lights there are working fine this doesn't seem to be the issue.
2) A bad/dirty connector(s) - Not sure the best way to test for this, but I could check the voltage of each of the pins and see if I can figure out which pin supplies power to the brake lights and see if it's getting 12+V. Does anyone have a wiring diagram for the tallight connector? I could try cleaning the connector, but if that doesn't work, are these replaceable? I could also check the connector/pins at the 12V battery, but I'm not sure which one supplies brake light power.
3) Bad LEDs or wiring in the taillight assembly itself - It sounds like sometimes the LEDs simply just fail but given that all 6 are still lighting up (and just dim) I suspect it's actually the connection that's bad.

Any ideas/suggestions? I don't know much about electronics, and I'd like to confirm I'm on the right track before I try ordering a new taillight assembly or something. I was thinking I could try plugging the driver side connector into the still-good passenger taillight and see if the issue persists there or not, but maybe there's an easier way to do things.

 

So I was able to measure the voltage on the brake light pin (the blue wire on the connector) and verified the ground it's using is just as good as the 12V battery ground, it's getting identical voltage to the brake light on both taillights (10.6V). I suspect the issue may just be the weak 12V I've been meaning to replace for months (it only shows 11.5V at rest these days), but that still doesn't explain why the passenger-side lights worked just fine with the same voltage (maybe there is a voltage drop when it gets to driver side?). I guess I may as well go ahead and replace the 12V with a 55Ah PriusCamper recommended a while back to rule that issue out.

@Tombukt2 I'll try cleaning the contacts with some MAF sensor cleaner I have on hand to see if that helps.

Unfortunately, something went wrong during my multimeter testing and the car is no longer going into ready mode. It goes into accessory mode only (green, orange light), but there's no red triangle or any other trouble lights. I tried unplugging/replugging the 12V to reset the car but the issue persists. I hope I didn't blow a fuse while trying to measure current :/ The brakelights don't seem to be getting power on either side now (but hazards are fine).

 

Hmm, I wonder if I borked my brake light switch or pedal while holding it down with a pipe, since that would explain why my Prius won't go into ready mode. I can hit the power button once and twice and it turns green/orange with everything turning on as normal but it refuses to go into ready mode.

Most things electrical are powering on as normal - MFD, dashboard lights, headlights, turn signals/hazard lights, taillight (for nighttime, but not brake light).

I just cannot get the brake lights to light up at all anymore. I'll check the #9 STOP fuses in case I've blown those: Fuse Box Diagram Toyota Prius (XW20; 2004-2009) I'll also check AM2 just in case that blew somehow.

 

Just tried to do a jump start but could only get into accessory mode. I tried the "emergency start procedure" mentioned here: Prius won't start...ready sign never turns on | PriusChat but couldn't get it to work, probably because the brake switch isn't working?

The brake pedal itself *feels* fine, nothing seems to be stuck or unusual as far as I can tell. I also tried pushing *hard* with my body weight and pumping it a few times, but still nothing from the brake lights and no ready mode. All other lights/electrical systems seem fine in accessory mode so I wonder if something went wrong with my brake switch just now...

Any pointers for adjusting the brake pedals or diagnosing/fixing problems with the brake switch? I saw this thread re: brake switch failure: Brake switch failure, no start, no brake lights | PriusChat It sounds like I could even manually reach under the steering wheel to tap it on if needed? Brake light switch button stuck; brake lights won't turn on; any quick fixes? | PriusChat I hesitate to mess with this for safety reasons, hopefully the issue is just electrical. I guess a blown #9 STOP fuse would also prevent the car from going into ready mode?

 

So the #9 STOP fuse (7.5A) was indeed blown.

Any recommendations to get spare "mini" fuses? It looks like they're a strange proprietary size... Type "A" Fuse? | PriusChat EDIT: bought some Littelfuse replacements and a pack of mini-ATC fuses.

Unfortunately the fuse box only has spare 15A and 30A fuses. I don't see a 7.5A fuse I can temporarily use except maybe #3 DRL (which my car doesn't have?) or #8 OBD (hopefully nonessential if I don't have codes). I could temporarily use the #4 fuse (15A, rear wiper) but don't want to risk damaging my electrical system. EDIT: ended up using the OBD fuse for now.

Any recommendations for replacement taillight assemblies? Some aftermarket units seem to have fit and longevity problems:

 

So I plugged the bad taillight into the other side connector and saw the same dim LED issue, so something on the taillight assembly itself (either connector or LEDs) is definitely the culprit.

EDIT: I tried cleaning by spraying a ton of QD Electrical cleaner but it didn't help. I also wiggled the connector and tried pushing the pin on the blue wire deeper but it didn't help. So it looks like the connector itself is fine, must be something wrong inside the assembly.

I could also try de-pinning and re-pinning the blue pin with a jewelers screwdriver to see if that helps:

LED brake light replacement | Page 2 | PriusChat But I'm thinking it's probably bad LEDs inside.

 

I did try testing the light with a jump pack connected and with a 3A battery tender but the left side was always dimmer. I do have a new 55Ah battery coming anyway, so I could hold off on a new taillight until then, hopefully the new 12V resolves the issue.

 

+1 to my_guy_mann I believe it was my mistake while testing the stop lights.

I was able to swap out with a new 12V battery (12.6V at rest) but the driver-side headlight is still dim. I'm going to give an aftermarket unit for $75 off Amazon a try:

It sounds like they aren't the best but hopefully I can get a few years out of it.

EDIT: On second thought, it looks like OEM is only $142: https://parts.lagrangetoyota.com/productdetails.aspx?modelYear=0&stockNumber=8156147071&ukey_product=63113460&referer=parts.toyota.com&machineIDT1=3iikuhgor3wh2dpuvaqylbrf