Hi,
When you say you hacked the DMS I assume you mean that you hacked the Phatbox (or Digital Jukebox in Volvo-speak). The DMS is merely the means of getting the hack into the box.
Opening the Volvo DMS cartirdge is no different to any other. After removing the four small screws, the case can be split by very carefully forcing the two halves apart. Use a fine blade, but do not push far into the case. There are also retaining clips moulded into the plastic, so take care to ease it open a bit at a time. I generally work my way completely along one side before starting on the other.
Re-assembly is straight forward, but take time to ensure that the spring that presses against the eject button is correctly located.
Good luck!