The jumper is generally in the left rear area of the engine compartment.  Here's a picture of the jumper on Tom Hollingsworth's '90:

Yes, there are two connectors.  The bright orange one has a pair of wires going to an idle microswitch.  The black one has a short loop of black wire.  It's the black one with the short loop of black wire; the key is reportedly to look for yellow wire, and on this car there is a yellow wire on the other side of that connector (not clearly visible in this pic).

Here's what the same jumper looks like on David Buchner's '94:

Notice that there's only the one connector here -- and the loop is yellow on this one.  The connector itself is different as well.

Regardless of what the jumper itself looks like, they all do the same thing.  With the jumper in place, the Marelli uses an advanced timing map requiring premium octane fuel.  With the jumper removed, the Marelli system switches to a more retarded map that allows the engine to run safely on lower octane fuel.  Buchner reports that the performance drops way off, as would be expected from retarded timing.  Hence, most people will find it best to leave the jumper in and seek fuel with sufficient octane, and only remove this jumper when they accidentally get a tankful of lousy gas.

Note that these engines tend to knock lightly even with good fuel, especially if they've been driven too gently for too long.  If your car is knockin', it is suggested that you perform an Italian Tune-Up on it before seeking other corrective measures including removing this jumper.  There are two steps to an Italian Tune-Up:  First, check everything else on the car to make sure it is in good operating condition, including the cooling system, fuel system, brakes, and tires.  Then, take the car out on the road and allow it to fully warm up.  At about 60 mph, put it in second gear and hammer it.  Somewhere around 5500 - 6000 rpm, let off the throttle and allow it to coast back to sixty.  Repeat several times.

People who perform the Italian Tune-Up universally report that the car runs much better afterward -- even if it wasn't knocking beforehand.  Some claim the first tromp on the throttle results in large clouds of black smoke out the rear as the crud clears out, but after a few tromps it's clean and the throttle response is much improved.