As the title states, i recently bought a 2012 XC60 t6 for my wife to daily drive. Recently we got a brake light bulb error on the dash so I went to swap bulbs only to find that the bulb is not the issue. I had previously installed LED bulbs in the brake lights for brightness/longevity but the left rear stopped working.

things i have done so far:

• swapped bulb with right brake light which was working, did not resolve issue
• checked bub for polarity due to being LED, did not work in either orientation
• replaced left brake light bulb with standard filament bulb, also swapped right side to standard bulb as well to rule out low current draw possibility sometimes associated with LED bulbs, did not resolve issue
• checked fuse #22 (5a) in fuse box B under glovebox, found to be in working order
• tested wiring on both sides with a multimeter, right reads slightly above 12v, left is reading slightly less than 6v at the connector

I have looked over the wiring and did not identify any physical damage to wiring or connectors.

any suggested course of action from here other than taking to the dealership? Is there a relay or some other common issue to check?

would like to resolve this soon and am willing to do the work myself with suggested items to check. I’ve had many VWs, Audis, and other cars with crap wiring over the years but never seen an issue like this before.

i have also done some online searching but havent identified anything that seems to be relevant to this model specifically.

 

Do you have 12v (with your $3 test light) at terminal 2 on both the left and right tail lamp assemblies? The violet and brown wires?

 

As previously mentioned, passenger brake light i’m getting 12v, driver brake light im only getting about 6v. I tested using a multimeter with the positive probe pressed into the harness connector and the negative probe to body ground, tested the same way on both sides. I did notice that the driver side brake light actually has (2) wires going to the positive pin connector, where the passenger side has a single wire for positive power. In both cases, yes the positive feed wires are purple with a brown stripe, 2 on the driver side and only 1 on the passenger side.

I did play around with a few things though and got mixed/confusing results. I ran a jumper wire from the positive of the right (working) brake light to the positive of the left brake light. After doing this, the brake lights both worked but only with the engine running and not wile the engine was off. And the brake lights would only work for about a minute before both would **** off on their own. Turning off the engine and restarting would make both sides work again but only for about a minute. I repeated this cycle several times with the same result. Checked the brake light fuse with the car off and was found to be in proper order.

I ran out of time and daylight so i will pick it up again tomorrow but for now i’m planning to switch the jumper wire to connect to the “other” positive wire for the left brake light after some further diagnosis with the multimeter.

Should I assume that one of the positive wires for the driver side is switched power and the other is constant with input from the brake light switch/relay?

 

The CEM may shut off power with both bulbs jumped together because it's using too much current.

There is no relay shown in the wiring diagram. Switch is 3/9, CEM decides when to send power either from a brake light switch signal or when brakes are applied automatically by city safety to the 2 vt/bn wires terminating at connector c2:59 and 60 on the CEM (also the ye/gy wire @ c2:9 for the 3rd brake light)

You might check where those wires terminate at the CEM - water intrusion is a problem - and corrosion can cause weird electrical problems.

 

Forgive my ignorance but where exactly is the CEM located?

also, there is no issue with any other light in either of the rear light clusters. It appears that the ground wire is on a loop from brake light to turn signal to tail light on each side so would it not stand to reason that if there was a bad ground it would be affecting all bulbs in the cluster?

3rd brake light is working fine.

i removed the jumper that bridged the two brake lights and am now back to square 1 with the passenger brake light working as expected and the driver side not working at all.

 

Cem is located above passengers feet