Finally got it sorted.
Ended up using a script task to msgbox out the value each time to ensure it was doing what it was supposed to.
Turns out it was.... which was VERY perplexing.
Recreated the project, now I'm thinking I may have screwed up by not setting the expression to set the connection string in one of the connectors that the solution was using.
Have redone the solution, it seems to be working now.
I think I must have either not gotten the entire expression in there, or blanked out one of the connection strings by accident. Either way, it is doing it's job! 🙂