Title says it all. Can't seem to get even a lick of power from the constant (Mazda Blue/Red, Stereo Yellow) connection. This is the only thing stopping my stereo from working and I'm stumped. I jumped the power (Blue/White) over to the constant line and the stereo turns on and everything's all fine and dandy, but without that I get nothing! What could be the issue? Thanks in advance.

 

Just so all bases are covered, have you checked for blown fuses? My memory is a little fuzzy on it, but I seem to recall the 7.5A ROOM fuse delivering power to the stereo's constant 12v wire. There is a 60A fusible link under the hood that feeds power to the Room fuse. If either fuse is blown or there is no continuity between their sockets, the stereo won't get the constant 12v it needs to operate.

The Install Doctor - The Do-It-Yourself Car Stereo Installation Resource - Quick Wiring Section - Radio and Stereo Wire Colors

If you follow the 83-92 Mazda wiring information contained here, it should work without a hitch. Constant 12v is the single terminal in its own one-pin connector. Switched 12v, Illumination+ & Antenna are all in the 3-pin connector. If you have a GXL or TII, you'll have one more connector than in the PDF diagram, a two-pin T-shaped one identical to that which plugs into the S4 alternator. The horizontal part of the "T" is the factory amps' turn-on signal from the stereo.

If the wiring checks out, I would say it was an error in splicing the two pigtails together. Fortunately, all of the terminals for the S4 are identical and not too difficult to extract from the housings if you goofed. Using the blue speaker connector on the stereo side (mates with the car's harness) as an example, orient it so the locking tab is at the top. You'll see a little ramp on top of each terminal. With a thin instrument (~1mm thick but wedge-shaped, 2mm wide), pick the ramp upward and pull the terminal out the back. In lieu of a proper de-pinning tool, the end of a pocket knife bottle opener may work but will require more effort to wedge under the ramp. As for reinserting them, push it in from the back untill it clicks. Needle-nose pliers can be helpful to seat the terminals from the mating ends by pulling, but I've not found it necessary on these ones yet.

On my most recent install, I left the illumination wire disconnected to maintain full backlight brightness at all times and spliced the stereo's remote turn-on wire to both the antenna and factory amps using an extra blue wire with male terminal, snapped into the correct housing (from a junkyard, they're super common on Japanese cars) for the amps' connector. My car is currently lacking an antenna, but I wired it this way to leave my options open in case I find a stock one on the cheap.