I have a 2003 Mercury Grand Marquis GS and have no Air Suspension Switch in the trunk on the left side like the manual says. Would it be safe for me to jack my car to change my brakes? and do an oil change?

 

If the car has the "air suspension" (air bags in rear), then the switch must be there.
To determine if your car has the air suspension, look under the rear of the car -- above the rear axle -- if you see coil springs, = no air suspension and no switch.
If you see rubber air bags, but cannot locate the shut-off switch --
You may jack up the car AFTER dis-connecting the NEGATIVE battery cable.
Dis-connecting the battery cable will prevent the air suspension compressor from running while you're working on the car.
The factory shop manual specifies that the NEGATIVE battery cable be dis-connected during ANY parts removal and/or repairs to the electrical system and/or it's components.