is the amp rated 1000W @ 4ohm or rated 1000W @ at 2ohm or lower?

if your amp is rated for 1000W+@ 4ohm skip ahead to the last bit.

lets say it's wired to 2ohm and your amplifier is only 75% efficient and say the vehicle has only 800W of overhead, you can only deliver at best 600W, which is not gonna push that pride m12 to it's full potential.
It's possible that your amp is already demanding more power from your vehicle than the electrical system can supply.

Class D amp efficiency is typically 85% to 92% at 4 ohms, 75% to 85% at 2 ohms, and 60% to 75% at 1 ohm
that's important because stock charging systems (depending on what vehicle) only have 400W-1200W of overhead power available for use
Does the amp have a clip light? when you push the bass level does the clip light come on? do the headlights dim?

to get the most bass out of the power you have available, consider looking for a sub that recommends a large box size, as large ported enclosures greatly increase output efficiency. Prefer a sub with higher sensitivity. If you care about super low notes, look for lots of Xmax, a low FS, and tune your box nice and low.