I'm replying in hopes that the next person attempting this can learn from my struggles. A volvo mechanic showed this to me. Crawl up into the engine compartment, like you're trying to look at the driver. Now, with a flashlight, look down on the firewall and you'll see a 15mm nut inside of a pie shap...

From the top of the engine, put a pry-bar between the plate, and the nut un the tensioner. It pops right off.