There is a 7 pin connector in back but I need to tow a trailer with electric brakes. Is there a n existing connector in the dash to connect a brake controller to or do I have to wire it in through the firewall?

Yes. It's tucked underneath the column. You also should've gotten the pigtail with your owners manual.

Even easier if you order the Toyota specific harness/adapter with your brake controller. Just plugs directly into the Toyota trailer wiring receptacle, up in there to the left of the parking brake pedal. Other end plugs into the controller. I didn't use the Toyota supplied pigtail at all - just plug and play!

Here's a pic...........

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Thanks for this info. I have a popup trailer with electric brakes. Those things make a HUGE difference!

And if you need info for the other end ---

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Where seems to be the best place to mount the controller? I purchased mine (along with the Toyota specific harness) a few weeks ago. I can't decide where it will be the least in the way. Anyone have any photos? I would greatly appreciate any advice.

My 4runner actually came with the Harness in the glove box. Summit racing has a bunch of controller options.

What brand did you buy, the Tekonsha P2 and P3 do not have to be mounted level and can mounted with Velcro just about anywhere.

I bought the P3 from E-Trailer. It never occurred to me to use Velcro, wat a great Idea! Thanks!

Thanks for the info. Is this wiring harness standard on all 4Runners?

I haven't bought the vehicle yet. I'm having difficulty finding a 2015 Trail in Super White in CA. No one seems to have one. I may have to get a silver one. I prefer white but silver will hide desert pinstripes just as well.

The harness from the plug under the dash to the plugs at the rear are standard from Toyota. The connector that goes to the controller comes with the 4runner this only has a plug at one end, but the best way if you buy a Tekonsha is to get the Toyota harness with the controller that has plugs on both ends.

From Tekonsha FAQ:
The P3 must be mounted in the direction of travel and should be fairly level from side to side. Front to back the control can be mounted at any angle.

Finding a place for the controller where it will be easily accessible, yet not in the way, is a bit of a challenge. I chose the Hopkins InSight Flex Mount for that reason.
The main unit is mounted out of the way, and out of sight, up on the wall just above and in front of the emergency brake pedal.
The hand control is just to the left of the steering column, on the dash. Easy to reach while driving, but not in any danger of getting hit by a knee.
The info screen is on top of the steering column, just below the dash instruments. Easy to see, without really taking your eyes off the road much.

You don't even notice the modules when you are in the car. All wires passed easily through the gap for the steering column - no drilling or cutting required.
Clean install!
Now, I'll try to add the photos! (I can't seem to get it to accept multiple photos?!)

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The info screen:

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