I have a 99 Tahoe when the lights are off everything works correctly. Turn the lights on everything works correctly except left turn rear both the brake and running lights blink opposite each other

Check your ground wired, The system is backfeeding.

You have a short or grounding issue . Viewing wiring diagram an testing the circuits with a DVOM - digital volt ohm meter doing voltage drop testing .
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Free wiring diagrams here http://www.bbbind.com/free-tsb Enter vehicle info. year , make , model an engine size . Under system click on lighting ,then under subsystem click on exterior lighting . Then click the search button ,then the blue link .

Check for a bad ground as all you have stated could be the result of a ground wire with a bad connection get back to me if you need more help Thanks Ray

maybe a bad earth on the left rear lights

check that all your bulbs are correct and are making contact. this commonly happens when a wrong bulb is installed in a socket. the other reason this happens is a bad ground, and one bulb uses another as a ground path. I would start at the rear left light and check that everything is good there, then move on to the front. 99 percent of these kinds of problems are at the lights themselves.

I had this problem, and to be honest, it happened to the left side about a week later...it's the board that the bulbs plug into...it's a very common problem with this model. So popular that you can normally get the socket board off the shelf at a parts store! it's extremely simple to replace, and I replaced all the bulbs at the same time!

Try having the dealer flush and bleed your system. It is very hard to bleed ABS systems yourself and have safe brakes that still work afterward. Bleeding non-ABS brakes yourself is easy not the same for ABS brakes. Valving, sensors and what-not require a tech and the correct equipment in my opinion. You do it wrong and you could ruin your ABS system. Do that and see if they firm up. I replaced my brake shoes/pads at the same time all new everything in back, drums/springs everything and new rotors up front. then I had the chevy dealer flush, refill and bleed system. Stiffer pedal and brakes work better. Keep in mind the brakes on 99 Tahoes are inaedequate, require new rotors often, heat up and fade/glaze pads regularly. I replace my pads long before they wear down because they glaze up and start fading early. I'll rough em up once maybe, next time, new ones. Every two brake jobs, new rotors for me. Just how it is. They will stiffen a bit and work better but they will never be awesome brakes. Just how it is on 99 and earlier Tahoes. Hope it helps. Very important to bleed correctly though. I'll bleed my 83 Toyota 4x4 myself but not the Tahoe.

sounds like one of the circuit bords at taillights is bad

Did you ever figure this out? I've got the same exact problem with my tahoe.

First thing i would do is chg. the break light switch, and make sure to chk. all grounds around tailights, sometimes if there is a ground problem it will travel back thru the hot wire to get a ground, witch in turn makes everything go wackey.