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This type of trailer wiring conversion unit is quite basic, but will do the job if you know how to hook it up. Tapping into a brake light at the rear will cause a bulb out condition every time you step on the brakes. In order to keep the bulb out sensor happy you need to run a wire up to the brake pedal switch to get ahead of the bulb sensor. You can tap into the turn signal wires at the tail light harness (left side). For the trailer park lights, tap into the the license plate lamp wire (also at the left harness) as this circuit is not on the bulb out sensor. You don't need to cut wires open, just use the crimp down block taps available at most auto supply stores. Make sure there is a good ground connection to the vehicle chassis. It is better to attach the ground wire on the trailer directly to the trailer lamp ground wire rather than the trailer frame as corrosion can often cause strange problems. On the trailer, use dielectric grease if you can to prevent corrosion at connections and bulb bases.
It's best to have a wiring diagram for your year and model, but tracing the wires by color is fairly easy especially with the aid of a multimeter and a pin to poke through wire insulation to confirm the conection point.
There is one problem with these cheaper units. The little logic units in these converters typically have so much resistance that you'll find the trailer lights are dimmer than they should be (the trailer lights may receive as little as 5 volts). For a little over twice the price you can buy relay driven units that properly power the trailer lights using battery power. Rather than attaching to the battery (as shown in their hookup diagrams) it's better to attach at the fuse panel to an unused or lightly loaded fuse that's on the ignition switch (for 240's fuse #10 is usually good for this -use a 16 amp fuse and make sure you connect to the cold side). For this type of unit it's best to run the brake light wire up to the front same as the cheaper unit, but the relay current draw is low enough that many people are able to attach the brake wire to one of the rear brake lights without triggering the bulb sensor. In my opinion, if you're going to all the trouble of running a power line up to the dash you might as well run two wires and do it right.
If you do much towing I would highly recommend spending the extra dollars for a relay driven unit (also known as a powered converter). I believe trailer rentals like U-Haul have them, but you can often get them cheaper at auto supply houses and RV dealers. Brand names for these include ModuLite from Draw-Tite, Tow-Power from Reese, LiteMate by Hoppy (Hopkins), BriteLite by Roadmaster, Max-Light, Calterm,... Prices are typically U$25-U$85
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Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day
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