Yeti, apologies for the delay, been out all day shopping for a pair of chairs with my wife, 28,000 chairs later she selected the first one we looked at, can you believe that?
Anyway, here is the photo of the plastic box and the cheap ebay 20amp temp controller connected to the fridge using an Anderson plug, plus because i have such enormous faith in my electrical ability, I have a portable jumper lead so i can bypass the controller and use the Engel controller if it blows up.
Basically, the controller has two screen displays, one is your setpoint and the other is the actual internal fridge temp gained from the temp probe. The internal setpoint I run my fridge at is 1.5DegC. To get the controller to do the work instead of the crude Engel unit, turn the dial up on the Engel to freeze or wherever is past your setpoint, plug it into the external controller using the Anderson plug and you are away. the external control now modulates the temperature. I would think you could do this with the waeco as long as you can get power to the compressor motor. If the electronics are shot in the control board, you would have to do some re-wiring to drive the pump i think.
Here is the link to the item from an Aussie seller, but I think you can get them a lot cheaper ex China but longer delivery period. If you search 12V fridge controller there are a number there including the ones I have used and also @TD, but I have had mixed luck with those lately - some faulty parts and some due to clumsy workmanship on my behalf also!!
http://www.ebay.com.au/itm/12v-DIGIT...oAAOSwuLZY2w0WSANY0034.JPG