You probably guessed right: fried MKIII nav comp.
Compare your symptoms to this post...
If your symptoms match, then you have a dead nav computer. If you have a 2001 with a widescreen display, then you have a MKIII nav computer. The MKIII is very sensitive to having its power disconnected before it fully shuts down. If they disconnected the battery without waiting for the MKIII to turn off, it is permanently fried.