It could be done, but not by just wiring the panel in parallel across the blower. The problem is that you don't want the normal blower current messing up your panel in some way, or vice versa, with electronics in the climate control, or heaven forbid, the ECU, getting blown up by the unexpected voltage from your solar panel. To be safe you could put a switch in the +12V line to the blower so that it only takes its current the normal way from the car in one position, or from your panel in the other position. But that would be inconvenient because you would have to remember to leave it in the proper position. Using a relay driven from some "only on when the car is" voltage would do the same thing, and you wouldn't have to manually flip the switch.