2019 Prius Limited Trailer wiring installation Problems

The Stop Light pages in the System Circuit section of the Electrical Wiring Diagram (more info) show that the wire harness design changed for 2019, presumably due to the styling changes to the rear light assemblies.

From the supplier’s photos of the Curt C56353 wire harness, it seems that it was designed to be connected in between the two halves of Toyota wire-and-wire connection PM1, but that connection exists only on model year 2016–2018 Prius cars.

For 2019 and later, there is a similarly-shaped wire-and-wire connection, now designated PN1, in the same general area, but the terminal assignments are different. For 2016–2018, stop light supply (blue wires) is PM1, terminals 12 (LH) and 13 (RH), and ground (white wire with black stripe) is PM1, terminal 1. For 2019 and later, stop light supply is PN1, terminal 13; and ground is PN1, terminal 4; the wire colors are the same.

You or your mechanic could rewire the adapter harness to intercept PN1-13 and PN1-4, while preserving continuity of PN1-6 (taillights) and PN1-2, PN1-3, and PN1-7 through PN1-10 (parking sensors).

You may already be aware, but Toyota recommends against towing a trailer or installing a tow hitch on fourth-generation Prius cars sold in the U.S.; see page 239 in the Owner’s Manual (PDF).

 

Neither the Toyota warranty nor an extended service contract would cover problems that arise because of modifications or damage to the car.

What I meant, however, was that the Curt adapter harness could be modified, before connecting it to the car, so that it would work with a 2019 model, without having to modify, tap, or splice the car’s original wiring.

You’ll notice that the Curt harness has two thirteen-pole connectors, one with male terminals and one with female terminals. These terminals—and the rubber plugs for unused cavities—can be removed and reinstalled in the field, though it’s a bit tricky, since the terminals are so small (0.64 mm or 0.025" wide).

Toyota’s Wire Harness Repair Manual (PDF) explains how; see the general information, the instructions for 0.64 waterproof type connectors on pages 31–33 and 55–57, and the terminal charts for housing part numbers 90980-12326 (page 44) and 90980-12716 (page 191). The manual was written with the assumption that you would be splicing a repair pigtail to an existing vehicle harness, but in this case, you need only to remove terminals or plugs from the connector housings and reinstall them in the correct cavities.

 

The Curt C56353 does not work for my 2021 Prius LE FWD: The turn signals work, but the tail and brake lights do not work. The reason: The lines spliced in the Curt tail/stop light T-connector go to empty pin locations in the Toyota connector. To make this adapter work you must move these splices to the pins shown in the attached figure.

 

Here is an expanded file documenting the Curt C56353 and the 2021 Prius LE FWD.

 

I gave up on installing CURT 56353 on my 2021 LE FWD. An apparent conflict with the pre-collision system generates malfunction alerts and disables systems like cruise control when the Prius is operated with its headlights on. See attached PDF.

 

Eleven months after I purchased a Torklift 2 inch Eco Hitch (X7404) and Curt plug-and-play trailer wiring harness (56353) from eTrailer for my 2019 Limited I finally got around to installing them. This was my first time removing the bumper cover on a gen 4 so the video links at eTrailer were very helpful. The hitch installation was straightforward, and I am content with its robust construction as well as the final appearance. The Curt wiring harness proved not to be plug-and-play for this model. I had the same experience as others with turn signals functioning but no running or brake lights. I confirmed the configuration of my wiring matched what Elektroingenieur and ramer described earlier in this thread. Then I spoke with a service tech at Curt in order to get some assurance that it was unlikely I would bugger anything up. I was willing to modify the Curt connectors but not the factory stuff. Since I was unable to disassemble the Curt 13-pin connectors to rearrange the pins, I resorted to the more crude process of cutting and splicing the wires between the Curt connectors. Specific steps are described below.

I am happy to report that everything works perfectly with no warning lights or diagnostic codes resulting. I think the difficulties experienced by ramer may have been due to running that circuit without a powered adapter. I wanted to avoid the fate of my son frying the Body Control Module (BCM) in his 2014 Volt by running LED trailer lights using a non-powered wiring harness.

A big thank you to Elektroingenieur and ramer whose posts led me down my successful path.




1. Cut the white/orange wires an inch away from where they join the red wire of the Curt harness.

2. Cut the #6 pin white/green wire midway between the Curt fe/male connectors

3. Splice together the white/orange wire extending from the red wire on the Curt connector with the #6 pin white/green wire on the female Curt connector along with the white/orange wire from the Curt male connector

4. Cut the white/brown wires an inch away from where they join the brown wire of the Curt harness.

5. Splice together the white/brown wire extending from the brown wire on the Curt connector with the #13 pin white/green wire on the female Curt connector along with the white/brown wire from the Curt male connector

6. For tidiness I connected pin 5 female (white/orange) to pin 6 male (white/green) and pin 12 female (white/brown) to pin 13 male (white/green)

 

I had the same problem on my 2019 XLE trailer harness in which the Brake and Running lights would not work with the original Curt harness kit. Thanks to this awesome forum for people posting the cause of the problem and how to fix it. Very much appreciated.