yeah sounds like you need load resistors. You could do the trailer but that only helps that one trailer, another alternative is to use an adaptor on the plug with the load resistors, so that acts as the middle man between the vehicle and any trailer. Seems less fuss that way ;)
http://tiny.cc/51klay
enquire on that one if it's suitable for your needs.
looks like someone has done similar already:
http://www.fordforums.com.au/showpos...&postcount=581