Have problem with the trailer hook-up wires. Have 7 wires just need to hook-up to the 4 wire prong. Which wires do I use? Have power to the smaller wires but not the big gauge black or blue wire. Only 2 of the wires seem to be hot?

Yes. Yellow-Left turn Green-Right turn Brown-Tail lights
White-Ground

if there is a power problem then the corresponding lights of the vehicle will out as well. If they work check where the harness is plugged in and check the connection.

The 4-wire flat plugs usually have a white (ground), green (right brake/turn) yellow (left brake/turn) and brown (taillights - left and right). If your brake and turn lights work but the taillights don't, check the plugs (trailer and truck sides) for corrosion, and chek both the trailer and truck wiring harnesses for chafed insulation and bare conductors on the brown wires. Likewiae if the taillights work and the brake/turn lights don't, perform the same checks on those wires. And last but not least, be sure that the white wires (truck and trailer) are grounded well to the truck body and trailer frame.

some van don t have a ground wire. hook up to trailer put jumper cables to frame of trailer and van if works install a ground wire plug

normally red is positive and black is neg,, but heres how a 4 prong should be hooked up correctly http://www.accessconnect.com/trailer_wiring_diagram.htm notice that the wires on a 4 prong going to the trailer are Green , Yellow, Brown and White,,

The 4 prong plug will have 5 wires attached to it.
The white wire is the ground and is attached to the trailer frame near the front. There will be two brown wires and one goes down each side of the trailer. These wires will have a connection at the side marker lights near the front of the fenders. They will then continue to the tail lights. The light bar in the center is also connected to the brown wire.
The yellow wire at the plug is for the left stoplight and the green is for the right stoplight. One of the wires out of the taillights will be brown and the other wire will not match the color of the harness wire.

With 4 prong and 3 extra, that will give you what you need for 7 prong for towing RV. If your not too experienced in that area, a mechanic at any trailer place, or hitch installation place or who knows RV wiring, should be able to do it for you in about 1/2 hr. or less. Testing the wires from your truck for what they do as in right , left, stop, running/taillights, etc is about the hardest part, and the 7 prong usually comes with what terminal does what.

if you go to the auto parts store they will carry one that you can un plug your harrness and put it inline

the back plug in on the truck most of the times on gm trucks has the wiring diagram printed on them

brown=running light
yellow=lt turn
green=rt turn
white=ground

your auto parts store also has a quick t plug that gives you a plug and play option/ or there is also a 7flat to 4 prong converter both are under $20

the wire you maybe talking about could posibly be for the trailer brake controller, which you have to buy serperate i'm not to sure how much it will run you but i would look into that because that is what the extra wire is usually forwith the 7 prong trailer plugs