Is there an explicit proof-text?
No. There is nothing in the Bible that says "Thou shalt interpret the unclear in light of the clear" or anything like that.
Are proponents reasoning from proof-texts?
Yes. Those who hold to this method of interpretation are leaning on a set of proof-texts. Here is the chain of logic:
The chain begins with the fundamental conviction that the Bible is the word of God. The following proof-text is often cited as the basis for this belief:
All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, --2 Timothy 3:16, ESV
The next link in the chain is that being the word of God, the Bible is fixed in meaning. The following proof-text is often cited as the basis for this belief:
But know this first of all, that no prophecy of Scripture is a matter of one’s own interpretation, for no prophecy was ever made by an act of human will, but men moved by the Holy Spirit spoke from God. --2 Peter 1:20-21
The next crucial link in the chain of reasoning is that being the word of God, and having fixed meaning, the Bible is true. The following proof-text is often cited to support this belief:
If he called them gods, to whom the word of God came (and the Scripture cannot be broken), do you say of Him, whom the Father sanctified and sent into the world, ‘You are blaspheming,’ because I said, ‘I am the Son of God’? --John 10:35-36
From here some basic logic can be employed: if two passages appear to be in conflict, and one is clear while the other is not, it must be assumed that since they are both true, the unclear can only have a meaning which is compatible with the clear. This limits the range of meaning the unclear could potentially have, and the end result is that you interpret the unclear in light of the clear.
Should the plain text be used to explain the confusing text?
This is where it gets a bit tricky (not to mention controversial.) We need to be very careful about reasoning this way, because in real life, people have theological presuppositions and frameworks through which they filter Scripture. So what often ends up happening is that two people will approach the same two passages, Person A with Theology A and Person B with Theology B. What often happens is that Person A considers the first verse "clear" and the second "unclear" while Person B considers the first "unclear" and the second "clear". Let me illustrate with a real-life example.
Many Calvinists believe that humans are 100% evil through and through ("total depravity") and therefore are unable to seek God. They would cite Romans 3:10-11 as clear support of this. Thus, God must supernaturally regenerate them by giving them His Spirit before they can seek Him. Many Arminians, on the other hand, believe that God gives His Spirit to us only after we seek and respond to Him. They would cite Acts 2:37-38 as clear support for this. To the Calvinist, Acts 2:37-38 is more unclear, and so they might try to come up with a scenario where these Israelites had already received the Spirit prior to feeling convicted, asking what to do, repenting, getting baptized, etc. On the flip side, Arminians, when confronted with Romans 3:10-11 would consider it more unclear, and would proceed to try to explain it away as an "[over]generalization", etc.
My point is, interpreting the "clear" in light of the "unclear" quickly becomes an exercise in subjectivity, as each interpreter has their own ideas about what is "clear" and what is "unclear". (In many respects I personally would consider Revelation more clear than Ecclesiastes, for example.) After seeing enough of these theological cartwheels, I prefer to just focus on doing good exegesis on each text on its own terms rather than trying to "solve" the perceived conflicts. And what I've found is that (in most cases anyway) once I've understood each passage on its own terms, through sound exegesis, the conflicts disappear.
Just for fun, consider the following exchange between Jesus and His opponents and see if you can decipher His method of reasoning. Is He challenging them to interpret the unclear in light of the clear, or the clear in light of the unclear, or both responsibly and assuming the results would mesh just fine?
Now while the Pharisees were gathered together, Jesus asked them a question: “What do you think about the Christ, whose son is He?” They said to Him, “The son of David.” He said to them, “Then how does David in the Spirit call Him ‘Lord,’ saying, ‘The Lord said to my Lord, “Sit at My right hand, until I put Your enemies beneath Your feet”’? If David then calls Him ‘Lord,’ how is He his son?” --Matthew 22:41-45