2001 Volvo S80 Brake Lights

Most likely problem is going to be your brake light relay that has gone bad. The relay is located on the rear electronic module located in the trunk behinde the left side cover. Check fuse #3 15A to make sure it is ok. Use a simple test light to make sure there is power going through it as well. If fuse is good replace the relay. If you look closely you will see a letter with a number. Look for RMI5 that is your relay that conrols your left and right side brake lights. You can swapp out the relay for the one above it just to test it really quick. Now if for some reason that is not the problem then also look at the very thin chips above the relay. Those are called shunts. I may have seen only one go bad but would not rule that out as well. The number location for that is RSH6. But like I mentioned before more probable cause is the relay. From reading your post it looks like you already now what you are doing so just pick up the relay yourself and install it if it ends up being bad. Good luck and hope this long post was not confusing. If you have any other questions just ask and I will try to answer it the best I can.

I do not see how the Brake Control Module would effect your brake lights since the lights are tied into the Rear Electronic Module. Swapp out the relay and check the brake lights. If that does not work swapp out the shunt. And see what happens. I posted the locations on the first post.