aha, now i get you! thats strange, have you got a headunit ready to go in? you could attach in a new block but personally I would just bullet clip or somehow connect the rear speaker wires direct to the headunit rear output wires of the aftermarket block, not the one in your pic, the one that it plugs into.
I only say this because I hate cutting the standard looms apart unless its absolutly vital.
and unless you get the connector block out of a golf of the same age ( although i think some did have 4 speakers but not too sure) you may have problems using your modded block into the aftermarket HU ( not standard golf HU) block, the wiring intputs may be in different places if that makes sense. they probably wont be ( especially if it came off a golf of the same age but with 4 speakers instead.),but like i said i would just cut the aftermarket block rear wires and attach direct. less work as well to be honest! and thats probably what has been done in the past i would have thought.
but it is entirely up to you.
hope all that makes sense.