1991 Montero and the brake lights and rear turn signals will not work. Do I need to replace a certain harness?

Richard, sounds to me like the turn signal Flasher is bad. You don't need a high dollar one. One for a few dollars will do the trick. I would replace the flasher FIRST as that is a common problem. Hope this helps you.

I had this problem before. I found the wiring harness at the rear light to have a short in the wiring. Certain ways you move the wires would allow it to work intermittently. After changing the harness, no more problems.

your problem is most likely the turn signal switch since your brake light circuit runs thru turn signal switch but would check harness going to passenger rear w/turn signal on check wiring with test light then maybe at the steering column to see if problem is in harness

Check the ground to the back tail lights, the ground may be poor and when the lights are on and may draw through the running lamps or vise-versa. Inspect the bulb connections and using a multi-meter or inspection light check for 12V DC at the terminals. Have a friend pump the brake pedal to check the brake lights to be certain the switch is ok (the switch is actuated by the brake pedal just under the dash)

I hope this may have assited you,
Tom

is the right rear turn signal doing the same if so it may be the turn signal switch

The bulb connector can be replaced, but the bulb you have may not be making good contact, try changing the bulb with the other side to see if it still don't work. Then you know for sure the bulb connector is bad. Test the contacts where the bulb goes in and see if power is coming to that point. A tester is cheaper than replacing the connector and the trouble you go through.

Oh boy, Chevy uses 3 important wires to make up their tail light harness. Brown is used for your parking lights and yellow and green are your signal and brake light wires, yellow one side, green the other. You have some seriously crossed wires. Start with tracing the lic plate lite since that works. Make sure it leads into your main parking lights and the ground wires from these tail lites aren't cut. The lic place lite will work with just power because the body of the lite is grounded with the bumper. Then make sure that there is power to the signal wires. You can do this with a test light and the signal light switched on. You don't need anyone to step on the brakes since it's the same signal, only interepted. Most importantly, Remove the tail lites and check grounds!! Good luck

Two issues here, you will need to replace the flasher. this will correct the erratic turn signals.

Concerning the brake light. Make sure the bulbs are in working condition and, check the wiring harness for faults as well. if the bulbs and wires arr OK, replace the brake light switch.

You can test for proper current by pulling the brake light bulbs out the socket and, use the copper prongs inside the brake light socket for testing. use a volt meter and, hit the brake pedal and, see if the meter registers a current. if not, replace the wiring harness.

Could be your turn signal switch!
The brake lights in this run through the turn signal switches!

I would Get in the Back pull up anything covering the Wiring harness for those lights and Look for A Corroded Connecter AND the GROUNDS on all thats why urn comes on while you Brake Electricity Finds the Easyest PATH to GROUND AND IF IT IS THROUGH THE Turn Sygnal switch GROUND is your FIRST PRIORITY here Trust me I'm ASE Certified in Electrical Systems
Grounds FIRST Please Dont forget to RATE me Thank You !