Far as I'm aware they are on the jts and connect the ecu to the lambda sensors, coil packs etc. It appears that some owners have soldered their plugs and this has improved the performance of their cars and stopped the ecu recording false codes.
I'm not sure the diesels have an equivalent. There are lots of large electrical connectors on the cam belt side of the engine. This area might have something you're looking for.
I doubt if a main agent will provide a cost effective repair. They will just change all the possible suspect bits mindlessly 1 by 1 at your expense.
Hold fire till you have your code reader. Cam sensor, crank sensor, slipped timing, broken injector, loose electrical connections, water in the diesel, compression loss, relays in front of battery, bad earth, etc. The code can narrow the list down.
Last edited by dgmayhead; 15-03-16 at 19:55.