Usually noise will enter the sound system thought one of two points, the power supply or the RCA's.

Seems some cars are prone to noise more then others, and Nissan truck are generally “quiet”.

It may be a good idea to see if you can try another amp (borough one), and hook it up. If the noise goes away it may be the MTX amp.

The easiest way to tell if it is come in on the power wires is to unplug the RCA's from the amp and put in a muting plug (a short RCA connector with both wires connected together), this will be an input of 0V to the amp. If the wine goes away then you now that the noise is entering on the RCA's and you do not need to do anything to the power wires. If entering on the RCA's you could try unplugging the ones that you have and routing another set straight from the deck to the amp (over seats and every thing) if the noise goes away you may just need to reroute the RCA's.

A lot also depends on if you have other stereo equipment in the vehicle.