I have a 1999 Chevy Silverado 1500 that i need to add an electric brake, i have the brake wiring and a factory harness, however i need to switch out the harness at the rear of the truck to a 7-pin, do i need to order that part from chevy and will the trailer lights and brakes work then?

You should have two Yellows
1999 Chevrolet Silverado Stereo Wiring Constant 12V+ Orange Switched 12V+ Yellow Ground Black Illumination Gray Dimmer Brown Antenna Right Window Trim Front Speakers 6 1/2" Doors Left Front (+) Tan Left Front (-) Gray Right Front (+) Light Green Right Front (-) Dark Green Rear Speakers 4" x 6" Side Pillar Left Rear (+) Brown Left Rear (-) Yellow Right Rear (+) Dark Blue Right Rear (-) Light Blue
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1999 Chevrolet Silverado Car Stereo Wire Colors and Locations

the12volt.com https://www.the12volt.com > installbay > stereodetail
1999 Chevrolet Silverado Stereo Wiring ; Left Front (+), Tan ; Left Front (-), Gray ; Right Front (+), Light Green ; Right Front (-), Dark Green.

Go to this link for wiring diagrams. http://bbbind.com/free_tsb.html

brake switch.located on the brake pedal.

In the 1999 Chevy Silverado 1500, the headlights are not designed to be on all the time by default. They are typically controlled manually by the driver using the headlight switch, although some vehicles may have a daytime running lights (DRL) feature. If the headlights are continuously on, it could be due to a modification or wiring issue, and it's recommended to consult the vehicle's manual and seek professional assistance if needed.

if its set up for trailers with brakes, it should have small black box under the dash above the pedals; if there is one you can adjust the brake timing to syncronize the trailers brakes with the trucks; if there is not one you can purchase one and mount it and run a few wires

Northen Tools sales a converter for it,

check fuse under hood, batery terminals coroded wire at harness if everything fails run a hot wire to rear brake line with an interuptor.
check the swich under the foot brake.
vitally

think brake switcjh it is a dule part switch.. if bulb good then switch next culprit

off of your factory harness you will have a small brown wire and a small blue wire among the 5. Do not use the small brown wire, use the blue.

Have you Checked the Harness under the Rear of the Bed for the Tow Package? These Truck usually have the Plug Harness installed from Factory and can sometimes come Undone.