hi,
the ecu is located in a plastic box near the right front wing next to the strut tower. but if you have an auto trans there will be 2 ecus in there (1 for engine ,1 for trans). the engine ecu is usually the one nearest the engine,
if you email me i can supply a wiring diagram for the engine to you.
however to do the whole fault tracing thing you need to be reasonably competent with a multimeter and electrical circuits,if not then go see a auto electrician and give them the wiring diagram and explain what you want them to do.
if the MAP fault occurs all the time then i would recomend going to see your local volvo dealer and ask if they could try swapping a ecu of another car to eliminate your one,if your ecu is faulty then this would prove it straight away.
good luck.

steve.