mercury grand marquis Blower only works on high and randomly selects vents.
Blowers that only work on high usually from a bad blower resistor pack. Drop the glove box and remove the insert to get to the blower motor, the resistor pack sits on top of it and is easy to change. If you're getting "randomly selected vents" you may want to test the pin outs on the dash vent control with a voltage meter.
Search Mercury Grand Marquis Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Mercury Grand Marquis Experts