So, I've been reading. Because I have just purchased a plug and play radio from Crutchfield.com and let me tell you there's no plug and play at all.
To start out I plugged everything in. The aftermarket cheap 8 pin harness was horrible. So that's being fixed with products from Wirecare.com.
Overall it's the same issue. I bought the car with no radio installed. Got a radio to put in. And bam. Only the tweeters work.
If it's that aftermarket connection that's being fixed.
Fuses have been checked (voltmeter + visual inspection)
All connections have been tested (voltmeter and visual inspection for corrosion (western New York)) I'm still at loss as to why only the tweeters would work. Is there something I missed? What do you think the overall problem is?

 

Ford Mach 460 systems are highly unreliable. The amps and speakers are not an issue. Buy a new radio from Crutchfield and walk away. Crutchfield supplies free adapters, provides a wiring diagram and tech support if needed. HOWEVER, you need to be able to solder wires (or use wire connectors) and able to follow a wiring diagram. The new radio does not simply plug into the Ford connectors. There is a new Mach 460 radio for sale in Sothern FL for $1300.00 that simply plugs in.

 

If the bass (amps) do not work, you did not supply the 12 volts into the 5 volt converter from the radio to turn on the amps. Not the radios fault. If the 5 volts is active, either a wire or the amps themselves have an issue. I think the project is above your skill level, time to take the car to a stereo shop to complete the installation or troubleshooting fixing the amps.

 

Im having this problem now too, ive done everything