There are not! Lucky us right? Haha there are a couple options you can do though.
1. Get a new head unit and wire up the speakers to that (this is the best way in my opinion)
2. Splice them into the wires from the other speakers, this is the cheapest. (I did this in my car and have had no issues)
Our cars were set up to only have two speakers with the head units we were given, yet there is room for 4 speakers, stupid right?