theres only one wire, which connects to the little fusebox in the engine bay. the connectors just slide out.

afaik there isnt anything else that ties the looms together, the feeds to the coil / rev counter are part of the car loom, the engine loom only connects to the following,

alternator

oil pressure sensor

crank sensor

injectors

air temp sensor

lambda sensor

coolant temp sensor (ecu)

coolant temp sensor (dash)

starter motor

stepper motor

throlle pot

charcoal cannistor

one wire to the underbonnet fuse box

and connectors to the relay pack behind the ecu, and to the ecu.

i'm sure the internal looms are common through all (or most) models. the internal loom on my gti has connectors for central locking on the rear doors, and rear door switches even though there was never a 5 door mpi metro gti.