You're not going to be able to find a plug-in fix to your problem. Trailer breaks are mainly used on two or three axle trailers, the load is normally too heavy for a truck this size.

Here's what to do.
-Return the oem harness
-Buy anAtlantic British Harness $130
-Cut off the 4-way connector it comes with
-Install a 5-way connector onto the atlantic brittish harness
-Route the 5th wire from the 5-way and tie it into the reverse light wire,this wire just needs to have power whenever you want to back the trailer.
-Solder, or butt connect and heat shrink all connections.

This will solve your problems.

I've tried the discoweb method with the wal-martconnector, and hadfuse blowing issues. Once I installed the Atlantic British part, it worked great.

 

that statement about the trailer and weight is far from correct.

My 23ft Mastercraft prostar 205 weighs in at 2800lbs and has a tandem axle disc brake trailer, and it does not exceed the weight limit of the car.

Ive also been required to make my own 5 pin adaptor, which was actually stolen, so thanks for the info as it has been about 6 yrs since ive made the adaptor =D

 

Since you already have the LR harness, you may want to do what I did: replace the bullet type connector with a regular blade connector so you can buy an off the shelf 5 pin adapter. You'll have to splice in the reverse signal as I don't believe the oem harness picks this up. I changed mine from the round pin to the traditional blade style and ran a hot from a relay at the battery and trailer brake controller in the cab in order to pull my camper and charge it as I'm on the road. Good luck.

 

I use this...plug and play...about $100...works great...5 min. install: http://www.roverparts.com/Parts/9439C.cfm

 

Just read this thread today when I was trying to figure out how to connect a UHaul trailer to my LR. Followed the instructions here: http://discoweb.org/trailerwire/index.htm and went to Walmart to purchase the kit. Part number is different now, but I was able to buy the kit which "Converts Vehicles with Independent Bulb Turn Signals to Common Bulb Trailer Turn Signal". The connector was right behind the passenger side brake light and I used the crimp type splices to connect the wires. Good to go. Great write up Greg!