Hello all, looking for a bit of advice, I've run into a strange problem with
my intall, and I can't seem to figure out where it's coming from.

Rough outline- MTX Thunder 4244 4 channel amp, getting fed from a Kenwood CD
deck. Fronts- Infinity Refrence 6402cfp Rear- Infintiy Kappa 692.5i

Problem- Whenever the engine is running, there is electrical noise that
varies with engine speed, but not with volume; it's just as loud with the
volume all the way down as it is all the way up. OK, I think, bad ground;
sheetmetal screw in the trunk floor was pretty bad. Relocated, used a wire
brush on an angle grinder to get it to bare metal (power tools are fun!).
No change. It SOUNDS like it's coming from the rear channel only, but it's
not affected by the fader controls or gain settings.

On a whim though I tried swapping the RCA's on the back. The problem ONLY
manifests in the rear channel; if I disconnect the rear, problem goes away,
if I send the fronts to the rear, problem is still in the rear.
Disconnecting the rear RCA lead and setting the amp to 2 channel input still
results in noise in just the rear channel.

Could this be an internal problem with the amp itself, because, frankly, I'm
at a loss to figure this out. The rear speakers come with an external
crossover, if I disconnect the tweeter, the noise all but dissappears.
Could the external crossover be the problem? Or would it just be filtering
out the noise to the woofer?

Any ideas would be greatly appreciated.

Thank-you,
Mike O'Malley