Instrument Voltage Regulator 68 Cuda

That is NOT a horn relay, it's an IVR for a "non-rally" dash. Notice the prong on the end of the case, which is ground, hooked to the case.

Who knows if it's the old mechanical kind or has electronic guts? IF it actually is electronic, and IF it is decent quality, you CAN hook it to the fuel gauge. The fuel gauge has THREE terminals. ONE terminal is the 5V OUT to the other gauges, temp, oil if you have one. If you disconnect the old regulator in the gauge to prevent shorts, you can hook the new IVR to the 5V terminal on the fuel gauge, and that should automatically "jumper" the 5V over to the other gauge(s)