Gm Brake Harness | etrailer Designed for late-model GM trucks that do not come equipped with brake control wiring harness ports, this adapter plugs into your Hayes electric trailer brake controller at one end and butt-connects the pigtail end of the adapter to your vehicle's factory brake control harness wiring, and then plug the other end of the adapter into the Amazon: GM trailer harness Amazon: GM trailer harness. Reese Towpower 8506911 Brake Control Adapter Harness for GM Models. 4.4 out of 5 stars 80. $4 $ 4. 95. FREE Shipping. Tekonsha 3025-P Brake Control Wiring Adapter for GM. 4.7 out of 5 stars 163. $9 $ 9. 90 $15 $ 15. 28. Get it as soon as Wed, Mar 27.