@Vincent Plant,
They are assigning a contract to you. You MUST be able to see the contract. There's no way to accept assignment of a contract without seeing it. What if the contract also said, "Hey, we also get your first born"?
The only reason the wholesaler would be hesitant to show you the contract is if they're afraid you're going to back out because you don't like how much money they're making on the deal, or they're afraid you're going to look at the contract and then go around them.
First, make sure you're happy with the deal regardless of what the wholesaler's going to make. If it's a good deal for you, it's a good deal for you. If it's not, then it's not. If the wholesaler is making $1,000, I'm sure you'd be fine with that. If it turns out that the wholesaler is making $50,000, you need to be ok with that as well. After all, it doesn't affect your deal once you've determined it's a good deal for you. Once you adopt that mindset, let the wholesaler know this and then stick to it.
Second, if the wholesaler doesn't trust you not to make an end-run once you see the original contract, then that's on them. They have the house under contract. If, after seeing the contract, you decide to do an end run, you can't because they have it under contract. If they're still worried, you can also offer to sign an non-compete agreement.
That said, it's really on the wholesaler. But if you want the deal, you can help them over this hump.