Left brake light doesn't work - 2001 Jeep Grand Cherokee

Check the bulbs with a test meter, or swap left to right and right to left. Check connectors and wiring and in particular grounding connection on that side of the MYSTERY vehicle!

'year make model' in all questions please.

https://www.google.com/search?q=Right+Brake+light+and+right+turn+signal+doesn%27t+work

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if someone added an aftermarket item to your lights, they may not work correctly, have it looked at by a professional who does that type of thing for a living.

Look Behind the triangle panel on the left side of the bike. Next to the battery. FUSES

Chances are that is the bulb. Its a dual filament bulb. The brake portion of it is burnt out. try swapping it with the left side and see if it works then. If it works, get a new bulb.

take a test light and make certain your getting power to the socket. could be a bad socket

If you have any extra fuses, try replacing the ones for the head lights and tail lights. Make sure the fuses you replace are the same color as the ones you replace them with, that way the voltages are the same. Another issue is that a flasher could be going out.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/erik_3934b541f17c4e99

Hello, this indicates a problem with either the left rear bulb or the socket that the bulb goes into. remove the rear bulb and check, also take a good look at where the bulb plugs into the socket and make sure there are no corrosion or burn marks

First let's determine which brake lights are not working. Does the high mounted brake light work? Do both left and right brake light not work? If none of the brake light work, the problem could be the brake light switch at the brake pedal OR fuse #12 in the underhood fuse panel.
If only the bake lights at the corner lamp assembly don't work, check fuse #34 in the rear fuse panel.
The module your are referring to is the circuit board located in each of the corner lamp assemblies, check on website www.compnine.com for the part number of this item. I don't believe this is your problem though.

It has a bad ground to the left brake light bulb.

First off, the brake light circuit is fuse 13 in PDC to brake swicth to rear brake bulbs, there is no physical connection to head lighrts in that circuit. all headlight functions are done by hard wire to the BCM,(body controll module), its locatted behind and attached the JB, (junction block) at the left side of dash, I've seen moisture problems cause alot of problems in these 2 modules, Its alot of work to remove these modules, dash componets need to be removed first,More info on this problem would help. GOOD LUCK, RDJRP