Car sub not working

It's quite simple, positive from amp to positive of speaker, negative to negative. Make sure it's getting sub frequencies only and not full spectrum signal. Next (if everything checks out so far) see if the box dimensions and ports are correct for your sub. The sub and box combo will have lots of factors when considering tuning of the box and port size. You can find all sorts of info on the website for your sub. Qtas, max excursion etc etc etc. They might even list a few total volume and port dimension sizes for the sub you have and your taste in music. It's quite a science to get it right. More than buying a sub and buying/building a box to get great performance. When you get it right though... music to your ears (and chest)

You seem to have everything in spec. check your crossover points and your speaker wiring. subs may be playing out of phase.
A huge factor is subwoofer enclosure ...if the enclosure is not in spec with sub requirements the subs will not peform great.

Is the amp switching on? There are two 12v wires that have to be connected. One from a direct constant 12v supply source, and the other one from the ignition switch 12v source.Find a single 12v. supply which you can take from the lighter connection and then trace a 12v wire that runs from your ignition switch. This is mentioned in the amp manual.Double check. If the amp is coming on but no sound, then your wire connections from the amp are not correct.Or you have also connected the speaker wires incorrectly.Remmember not to connect the earth of the speaker wires to the direct earth(body of the car) but to the earth wires that are shown in the manual as this could blow the amp power IC. The earth wires of the speakers are marked -- and + respectfully and are also colour coded.You should have a RCA female plug connector from your amp to the subs. Check everything double and if still no luck their is a possibility that you could have blown an IC in the amp from wrongly connected wires. Hope this is not the case.

Fuses?

This is an internal power supply failure - most likely a failed capacitor or bad ground trace..
Your amplifier requires repair.
To verify this - remove the RCA cables and if the "heartbeat" is still present - take the amplifier to a local repair shop that specializes in car audio and tell them your symptoms.

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Hello,

I've got a pair of these MTX's subs powered from a Fusion 450 watt 2-channel amp. For the price, they are great. By the way, they are 4-ohm.

My first suspicion would be that something in the first incorrect wiring damaged either the amp or the subs. Can you explain specifically what the "installing the subs incorrectly" means? Was the amp output shorted out at any time or was it operated without a load? Either of these conditions will fry an amp and sometimes, the protection circuitry doesn't work either.

First make sure that your amp is still working. Check both the primary power fuse, usually located near the vehicle battery, and the fuses on both sides of the amp. Make sure that when you turn the system on, the power light comes on steady (blinking is protection mode). Test your subs one at a time on a speaker output.

Hook up the remote wire from the head unit (grey w/ orange stripe) to the power antenna wire (blue) on the wiring harness adapter.

you have the remote amplifier turn on wire hooked up wrong. The power antenna goes off when the radio does... so it will lose power when you put in a cd or go to aux.
install the blue wire labled AMP. or REM.
Problem solved.

remote wire turning on amplifier?
Ground for amplifier?
Gain improperly set?
Radio output crossed over above tuning freq. of enclosure?
Just suggestions. Not much info to go by.

here is a short test you can try, remove the rca audio cable from the amp & connect a diskman or toher audio o/p device that you can hook up with the same rca wires, turn on the amp & see if you have sound. you can also remove the speaker wires from the sub & using multi meter read the ohm's of the sub, most subs are 4 ohm. Hope this help locate your problem