Let's assume for now that it is an original (and if it's not, you should get your money back), and the reason it's not working is because it was used with a different head unit. Use the Phathack DMS Tools in Tools mode to update the firmware on the DMS to whatever head unit you have.
I'd like to clarify this as I've seen this comment made on the boards here a number of times. If the PhatBox itself already works with the car it is in, there is no need to do anything with the "firmware" on the DMS. This "firmware" actually consists of two types of files - a program that is uploaded to the 8051 chip that talks to the head unit, and files on the DMS that the embedded Linux uses on the main ARM CPU. If the PhatBox is already working in a car, there is no need to re-flash the 8051 with the same code over again - it just takes time and confuses users when nothing happens but the green light flashing for 5 minutes, while the PhatBox disappears from the car stereo.
As of any given release date of the firmware, all of the files except the 8051 firmware are compatible with all* PhatBoxes and head units (that the firmware knows about), regardless of head unit. In fact, when PhatNoise was still updating the PhatBox, a number of us would collect the newest files and run them on PhatBoxes they weren't released for.
The only time you can experience problems is if the versions of the files on a new DMS are different from the files you're used to on your old DMS - they may exhibit old bugs (if your original DMS is newer) or change features (if the new DMS is newer). In a case like that, it is a good idea to have the same version on all of your DMS cartridges for consistency.
You might have a problem if you have one of the newer car models and DMS firmware that was released before that car model was supported by the PhatBox. Chances are it will play audio, but the controls may be screwey until an update.
Some DMS cartridges may have optional features like the AAC decoder or SSA plugin, but that doesn't affect the basic operation of the DMS.
*Note: certain versions of the PhatBox had different hardware (Ford comes to mind). I don't know if the above is still true when moving a DMS between one of those and one of the standard PhatBoxes. I think it does, but I'm not positive.
The above is a lot more long-winded than I intended. The gist of it is: It isn't necessary to update the firmware or hack the PhatBox when testing, and telling a user to do so will likely result in un-necessary confustion. Get the box working first, then update the firmware if necessary, then do the hack. 3 different steps, not all at once. It will make things a _lot_ easier to troubleshoot.