Only the left rear trailer light comes on when I apply the brakes.

You can buy small adapters that you plug into the car harness connector and allows you to test that the power is being supplied correctly.

In your case, I would suspect that the trailer wiring is at fault, rather than the truck's wiring..

https://www.google.com/search?q=trailer+7+way+plug+tester

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If the trailer has electric brakes that are powered by a separate battery, you use a 4-pole to 5-pole adapter and connect the extra wire to the truck backup light circuit.
4 pin to 5 pin plug for trailer lights The Hull Truth Boating and Fishing...

Standard wiring colors are as follows:

4-Way

5-Way

6-Way

7-Way

Green

Yellow

Brown

White

Red

Blue

Purple

Right Turn & Brake

Left Turn & Brake

Tail Lights

Ground

Backup Lights

Electric Brakes

Auxiliary Power
Green- Right Turn and Brake
Yellow- Left Turn and Brake
Brown- Tail Lights
White- Ground
Red- Reverse (Back Up) Lights
Blue- Electric Brakes
Purple- Auxiliary Power

It sounds like your green and yello wires are shorted together somewhere along the length of the harness. I would check the wires on the trailer first. Easier yet, if you have another trailer to connec or access to another to use for testing, connect to it and see if the problem repeats; if it does the trouble is in the vehicle's wiring.

Disconnect the trailer wiring and recheck the vehicle lighting. If it returns to normal, then obviously the trailer is the problem. Quite often the trailer plug loses its independent ground and it will back feed thru other circuits trying to locate one. Especially if its a boat trailer, water intrusion can cause havoc on the grounds due to corrosion. The under hood electrical center has circuits intentionally meant to be used for trailers. You CANNOT just jumper into the rear lights of the truck (tail, signals and brakes coming from the tail light connector). Because it uses a multi-function switch, the brakes and signals are actually the same circuit, and have to be wired separately for a trailer plug to operate.

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Trailer Wiring Diagrams 4 Way Systems

4 way flat molded connectors allow basic hookup for three lighting functions; right turn signal / stop light (green), left turn signal / stop light (yellow), taillight / license / side marker (brown) and a ground (white).

4 way tow vehicle side.
4 way trailer side.
5 Way Systems

Same as 4 way system listed above but adds a extra blue wire for brake signal or auxiliary power. Used on pop up camper trailers or utility / boat trailers.
5 way tow vehicle side.
5 way trailer side.
6 Way System, Rectangle Plug 3/4 inch by 1 inch 6 way rectangle connectors right turn signal (green), left turn signal (yellow), taillight (brown), ground (white). The red and blue wire can be used for brake control or auxiliary. Use on a small motorcycle trailer, snowmobile trailer or utility trailer. Can also be used as custom wiring on trailers with 3 light/wire systems. See "Tow Vehicle Taillight Wiring" below.
6 way tow vehicle side.
6 way trailer side.
6 Way Systems, Round Plug

Round 1 1/4" diameter metal connector allows 1 or 2 additional wiring and lighting functions such as back up lights, auxiliary 12v power or electric brakes. Note: The black (sometimes red) 12v and blue electric brakes wire may need to be reversed to suit the trailer. Check with a test light or VOM. Uses: horse trailer, travel trailers, landscaping trailer, car trailer, etc.

6 way tow vehicle side.
6 way trailer side.
7 Way Systems

Round 2" diameter connector allows additional pin for auxiliary 12 volt power or backup lights. Uses: heavy duty landscaping trailer, car trailer, boat trailers, horse trailer, travel trailer, construction trailer, etc. Check with a test light or VOM.

7 way RV flat blade tow vehicle side.
7 way RV flat blade trailer side.

Did yo check the fuse panel inside the truck its located on the drivers side dash , you have to open the driver side door, in there is a 10 amp fuse that is linked to rt trailer brakes and rt turn signal for the trailer, if the fuse is good there is a black box under the dash located on the floor on the driver side, just to the left of the break, thats where must of the plug in wire harness is for the trailer breaks!

I had a problem with my trailer light wiring, everything was working except one light, I used an OHM meter and traced from the socket back to the next joint and found I had a broken wire, I split the factory harness and repaired the wire, you may have the same problem, mine was under the truck left side above the frame

Check it with a test light. White-ground, Yellow-left turn, Green-right turn, Brown-tailights. Check it on the truck first. There are seperate fuses for just the trailer wiring in the fuse box under the hood. The Blue and Red wires are for Trailer brakes, don't worry about those.

Do u have a 4,5,6,or 7 way (wire) system? If it's 4 way it allows basic hookup for 3 lighting functions:
GREEN is right turn signal and stop light, YELLOW is left turn signal and stop light, BROWN is license, and side marker lights, WHITE is ground.

5way is same as above, but adds a BLUE wire for brake signal or aux. power. Its used on pop up campers, utility and boat trailers.

6way (rectangle plug) green=right turn signal, yellow=left turn signal, brown=taillight, white=ground, red and blue can be used for brake control or auxilary. Used on small motorcycle trailers, snowmobile trailers, utility trailers.

6way Round Plug, same as above, but blue is electric brakes, and black (sometimes red,) 12v; note:sometimes red/black need to be reversed w/ blue to suit the trailer.

7way adds an additional pin at plug in for 12volt power or back up lights. Whoa! hope that answers your question w/out confusing u. Let me know if u need more info.

I took the vehicle to Bosch, an electrical expert in Germany.