Just to add to my post those speaker kits are still sold but whether yours fit into them or alternatives mention on the thread. The kit look like this:
View attachment 35090
So the cut wire is on one of those loops. Basically the factory fitted speaker takes that loom plug that comes into it. There is no crossover in it just feeds a connection out that is then passed up to the tweeter. In the adaption kit they sold for VWs for other speakers that cross wiring to loop back in to the loom is shown above. Those red and black leads go into your own crossover in your case. To silence the built in speaker without digging the tweeter out you just snipped the loop. Reckon we are looking at the one by the bottom wire. 2 being the next one down. In your case you are digging the trim out... or the other end.
The Infinity speakers contained the built in tweet which is front mounted above the bass / mid range so the desire was to knock that one out. Think the kit you are installing has seperate tweeters so your leads you need to go into your crossover from the amp are two two end point leads. The inner leads feed the built in tweeter.
View attachment 35091
As I said this was years ago... probably longer ago than what I said. To find the one to disconnect I reckon I touched an ohm meter on it and listened for a click from the tweeter. At that stage the mid range / base speaker was unplugged. Looking at that socket in the photo above it looks like the two extreme ends of the plug connections are the ones that go forward. The inner ones feed the built in tweeter. The loom splits further up in the car to do that. You will I suspect be putting your own cable in for your tweeters from your crossover.
Trust this helps.