Dash lights dim when brakes applied or flash with left signal on

So Thursday night I noticed my dash lights started blinking with the left and left only turn signal. Also, when I apply the brake pedal my lights all dim. I am thinking a bad ground or short somewhere. Anyone ever have this happen?! 2010 Reg Cab 4x4. I do have a Curt plug n play trailer wire harness on it but it has worked fine until now, any possibilities this has something to do with it?! Maybe something went bad inside it?! Figured its tied into brakes and signal. I'm pulling her in my garage shortly to start going through everything just wondering if anyone else has heard of this?! Thanks

 

With lights dimming it would indicate a short circuit. The Curt harness would be a good place to start ( remove it and see if it goes away ). That being said it seems to be a problem with the Left Side Brake and Turn Signal circuit.

 

Bad ground some where.

 

Did you just replace any brake light bulbs? If not check the bulb contacts for corrosion.
Could also be the curt harness. Unplug it and check. I had one fail on me and kept shorting my tail light circuit.

 

If you have a short circuit it will blow fuses if you have a bad ground it will drive dash lights nuts. If it's just when the tail lights are on then that's is where you need to check first if it only happens with the signals that find out what side. Disconnecting the aftermarket harness may correct the problem that would be one thing to try but may not be the answer just because the problem goes away does not mean the harness t's self is bad it could be a bad ground at the other end. Factory wiring usually does not have problems but any time you see some thing spliced or taped up there is where you want to look. Toyota does not ground the tail lamp to the body in the back it gets to the lights through the harness so if a connection at that wire is bad it's going to cause problems.

 

So , here is what I found and corrected to fix my problem. Thanks everyone for the responses, however by the time I got on here and checked them I already solved the issue. I started by inspecting all wires, checked my grounds and since I was in my shop and not on the road I decided to check all the other lights. In fact, when I would turn my left signal on, not only did my dash lights flash, so did my front marker lights and all the LEDs in my light housings. When I applied the brakes, dash lights would dim out as previously stated as well as a slight glow from my front end lights. Everything else in the truck ( grounds, connections ) are good. I am anal with installation, always weatherproof, protect or mount to avoid rubbing etc. Anyways, it did indeed turn out to be the Curt wiring harness module. After checking everything else in the truck, I returned the tail lights to stock wiring configuration and problem was gone.My buddy is the manager of the local APS and delivered a new harness to swap out because I bought it from him originally. Interestingly enough, when I pulled the new wire harness / module out, it turns out that Curt totally redesigned the harness. Previously the module mounted in the center of the truck underneath and had two equally lengthed plug n play wires that ran up to each tail light with a short 4 way connection( maybe 2 feet long?!) to route to the trailer severely limiting mounting options to keep it protected from elements.
The new module / harness I received ( same PN / listed applications ) now mounts the module up directly behind the driver side tail light with a short ground wire and one very long set of wires to run to the passenger side and a considerably longer 4 way to route to the trailer......now its all nice and protected from the elements. Must have been enough of a problem for them to re design it

 

oh , and when I inspected the first Curt module I took off the truck it was "puffed" out in a few spots indicating to me that salt/water had penetrated it and caused damage / corrosion internally. Sad part is I just installed the hitch and harness in mid 2014

 

and this is why I need trailer lights....

 

Ha, you got snow! It have been a terribly winter up here right now we have maybe 6" of snow on the ground and a poker rally coming up this weekend doing a snow dance this afternoon to make sure we get hit by the nor'easter. I have only been out once this winter. The aftermarket controllers for the lights are not much with no real protection for short circuits if they are shorted it cooks a diode instead of blowing a fuse. That is an excellent place to put the controller my old 97's controller was in there also and lasted 285K miles.