Gosh Mike, you always tell people to do their own research, why don't you figure it out ?
None of the car makers seems to release model by model fleet vs. retail sales results. However, have a look at this, which is only one of many references to the issue you can find.
Fleets mask Big 3 woes Growing reliance on low-margin sales drains profits Bryce G. Hoffman / The Detroit News
"According to Murphy, Detroit's fleet sales reached new highs in the last three months of 2005, accounting for 29.5 percent of overall vehicle sales."
It is of course impossible to tell what the model mix is in those numbers or what the mix of daily rental fleet vs. other fleet user sales are.
If you travel frequently you will see that the Impala may yet dethrone the Taurus as the current king of the full size car rental fleet at the airports. Lots of Pontiacs out there in rental land as well.