I did this on a 2005. I bought this CURT 7-Pin Harness,
https://www.amazon.com/gp/product/B001EP0HPI/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1.
It plugs into your existing 4-Pin and then gives you the 7-Pin plus the 4-pin, But you do have to connect the extra wires. They give you butt-connectors, but soldering would be better. There are 4 additional wires you have to hook up. If I remember correctly, WHITE is Ground, BLUE is Brakes, BLACK is +12V and YELLOW is Reverse Lights.
White is easy, just connect to a chassis ground.
Blue should be there in the wire loom that the 4-pin wires come out of. At least is was on mine.
Yellow, you may need to connect up a relay with your reverse lights. I didn't hook mine up, and I don't think there was a wire in the loom for it.
Black, +12V, may be the hardest for you to get working. Someone else who has done a 3rd Gen might have more info on it, IDK. On my 2005 the +12V was ran back to a connector underneath on the drivers side between the front and rear doors. There is was an orange wire. I spliced in it there and ran a power wire back to the 7-Pin. I also had to add a fuse and a relay to get it working. And I highly recommend you get it working! I didn't at first, I did wire it up, but didn't add the fuse and relay. After a "short" 100 mi trip, approx 3 hours, with the trailer lights on and the brakes it almost completely drained my TT battery. My power tongue jack didn't have to power to lift my trailer off the car. I had to plug my TT into the house outlet to get onboard converter to give the jack enough power.
Making sure you have that +12V is just as important as the brakes. Your trailer battery should only power your brakes if it accidentally becomes detached from your tow vehicle!
On a side note, you should also have your break away cable attached to a different part of the vehicle than your hitch. If your hitch somehow fails and your break away is attached to it, how is it going to activate the brakes.
Good luck, I researched doing mine for a while before I found the info on the +12V.
