Can Trailer Wiring Harness be Connected to 12 Volt Power Outlet Instead of Battery on Toyota Sienna

Question:

Can the 12-volt power on a 4-blade vehicle to 7-blade trailer adapter be connected to a 12-volt auxiliary socket. My brother’s Toyota Highlander 4-way flat plug could not power the lights on my 2005 Aliner with a 7-way. I assume it was missing power. Can I provide power to the connection via an interior auxiliary plug rather than wire to the car battery. He only wants to borrow the trailer for a weekend. Part no.: 37185 Adapter 4-Pole to 7-Pole and 4-Pole

asked by: Jack D

Expert Reply:

The 4- to 7-Way adapter # 37185 will need to have the 4-Way plugged into it and the white wire grounded to the frame of the vehicle. The other wires (black for 12 volt power, blue for electric trailer brakes, and purple for reverse lights) don't have to be connected if all you want are the trailer lights to function. If you did want all of those functions to work I recommend # ETBC7 because it comes with the wiring and accessories needed to wire it all up so it is fully functional and so you can add a brake controller such as # 90885.

Four-way powered trailer wiring harnesses that require a direct connection to the battery are not tested or rated for any alternate installation method. Installing it by attaching the power wire to a power outlet 12 volt wire will void the warranty. Also many times those outlets are not the only component on the fuse and/or there could be a relay that has other functions. So if the wiring harness is drawing power and whatever else is fused with it or that uses the same relay is drawing power then it can cause more costly electrical problems.

You probably know already that routing the power wire to the vehicle battery isn't difficult; it just takes time. I have included a link to a help article I wrote that offers some tips for routing the power wire for you.

On powered harnesses, because they attach directly to the vehicle battery they have power all the time but unless something is plugged into the connector and drawing power it doesn't run down the battery.

If I was choosing a wiring harness for a 2011 to 2016 Toyota Sienna I would go with # 118498 because we hear about less problems with the Tow Ready brand. I have included a link to a video showing an example installation for you.