My pyramid 1000 watt amp only plays when turned all the way up???

Not to disappoint you too much, but the 1000 watts advertised is just marketing hype. If your amp is the PB781X, it should have only a single 15A fuse on it. Your battery provides 12.5 volts when the engine is off, so at most, your amp draws 187.5 watts of power. Draws, not outputs. No amp is 100% efficient due to heat. When your engine is running, assuming your alternator outputs 14.4 volts, your amp's maximum power draw is about 216 watts. Again, this is what your amp draws, not what it puts out. What you should focus on is the RMS (continuous) output, not Max or PMPO. Those numbers are useless, unfortunately.
Your amp is rated to output 50 watts per channel at 4 Ohms, and 75 watts per channel at 2 ohms. This is when wiring your channels separately, not bridged. I did not see true power specs for your amp while bridged, but it's possible you'll just get the full 150 watts to the single bridged output. Do not attempt to apply a 1 ohm load to your amplifier, as you will run the risk of damaging it. It's not designed to handle a 1 ohm load. If you're lucky, you'll blow a fuse before the amp is damaged, but either way, there is nothing to be gained by attempting it.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/reginald_bec291de192ca44f

1st have your subs facing inwards towards the front of the car, not facing out the boot, This creates a better bass.

Wire 1 sub to one channel and the other sub to the other channel.

Turn down your gain as low as possible and then power everything up.

turn volume to desired loudness and then start turning up the gain slowly, if distorts a little turn in back down a notch.

avoid using bass boost, I agree it sounds great but it puts more strain on your amp and shortens its life.

after sorting out your gain, tweak all others until sound is bassy and clear with no distortion.

If you would like to use bass boost, turn on bass boost first, then up your gain etc.

how much rms is the pyramid pb3818 5000w amp ?

Try contacting a local car audio professional or Pyramid customer support directly here:

http://www.pyramidcaraudio.com/ContactUs.aspx

Recommend contacting a local car audio professional or Pyramid customer support directly here:

http://www.pyramidcaraudio.com/ContactUs.aspx

Recommend contacting a local car audio professional or Pyramid customer support directly here:

http://www.pyramidcaraudio.com/ContactUs.aspx

If the sub is new, they need to be broken in.

All sub woofers need a break in period of about 5 to 6 hours at medium volume.
The reason is because the glue needs to be cured. This is done by the heat from working.

If you are giving it too much volume too soon, you are burning the glue.

Back off the subs and only play them at medium volume for about a week.

sounds like the Impedance of the speakers does not match that of the amp, therefore the output stage of the amp is trying to compensate, driving the amps too hard and eventually causing the thermal cut out to activate. Check the ratings on everything are compatible before you start sending it off to be fixed.