I'm just guessing without knowing which end goes to the trunk lights. if the lights go from the black trailer style plug to the interstate lights it is a 5-4 convertor.
The other way around and it would be a vehicle to vehicle tail light convertor. It takes your converted 4 wire signal-the trailer plug- and converts it back to a 5 wire signal. Here's a different one:
http://www.etrailer.com/p-118158.htmlYou have to follow the wires to know what style of convertor you will need to replace the defective one.
Not much help but it's all I got.
EDIT: The more I look at the picture it looks like a 5 to 4 convertor. The 4 wire connector with the extended hood is a bit unusual and is confusing me a bit.
It could be early hopkins 48915
http://www.etrailer.com/Wiring/Hopkins/HM48915.htmlor a calterm 5-4
http://www.ebay.com/itm/Trailer-Light-Isolated-Power-Supply-Converter-5-4-wire-PM32-/290660635436