First thing. Do you have a factory radio? Next, if you do, pull it out and see if there are three plugs into it. Whether it has gray plugs or black plugs, the subwoofer is the 8 pin plug and should have 4 wires in it. If someone has replaced the radio, you may not have one with a subwoofer. If everything is kosher, check for fuses and check that the enable circuit is working. Unfortunately, to get to the subwoofer and amp, you must remove that entire panel back there.
Darn near every Mach radio in these I've seen goes bad. The display goes out. So it's not unusual to have the radio replaced. One thing to look for to see if it's a true Mach radio is the clock button. Does it say RDS/clk on it? Or does it just say CLK? The first is a Mach radio. The second is not. But they can look identical otherwise. And most of the ones that aren't Mach do not have a subwoofer provision. Some do, but not very many.
