This exact situation happened to me (I matched to advanced position, but had to scramble for my prelim spot). Before match day, there is unmatch day. On this day, they send you an email. For those applying to both advanced and prelim spots, it can take 4 forms (wording not exact):
1. You matched to preliminary and advanced position
2. You matched to prelim but not to advanced
3. You matched to advanced but not prelim
4. You did not match at all
If you are in situation #2 or #3, then they DO tell you where you matched (the advanced spot if situation #2, the prelim spot if situation #3). This allows you to scramble with some idea of where you might like to end up geographically. As Dr. Cox points out, a great place to start looking for a prelim spot (if you matched to advanced but not prelim) is the place you matched for the advanced spot. Another good location is the place you're graduating from. In my case, these were one and the same, and that is how I got my spot in prelim medicine in the scramble.
Note: I matched in 2003, so this isn't exactly fresh information. You can (and should) confirm this with the staff at your medical school.