Engine noise: which lines cause it?

I have 4 engine noise related questions:

1.  I've seen reports that 12v-on amp remotes lines are too weak to cause engine noise but another that says he has seen it all too many times.

2.  I've also seen disagreement about which method is best for reducing the chance of getting engine noise when connecting a head unit without low-level outs to an amp:

A.  Hooking up the head unit's high level outs directly to the amp's high level ins.   OR

B.  Head unit's high level outs to a Line output converter (LOC) and from that to the amp using RCA's.

Beyond just engine noise, any opinions re: which is better (2A or 2B) for fidelity (distortion, etc.)  would also be appreciated.

3.  In case of 2B, since both high level outs and RCA's are involved, which should one choose to send the signal down the length of the car to an amp installed in the trunk:

a.  Head unit's line level outs (from the head unit all the way to the LOC placed in the back where it's hooked up to the amp with short RCA's)  OR

b.  Long RCA's from an LOC installed near the head unit.

Once again, beyond just engine noise, any opinions re: which is better (3A or 3B) for fidelity (distortion, etc.)  would also be appreciated.

4.   Lastly, is it true that one only need worry about engine noise induced on lines carrying sound to the amp, but not on the lines carrying audio from the amps to the speakers?

Thanks.

Thanks for the response, Satkunas.

Regarding your answer to 2A: did you perhaps mistakenly think I was going to connect my head unit's high level outs to my amp's LOW level inputs?  (I would never do that!)

Regarding your suggestion re: 3b, that ground-loop issue is something new for me to consider.  The ground-loop would be caused by my grounding the line output converter (LOC - a PAC-OEM2 in my case) in the back?  Hadn't planned on grounding the head unit anywhere at all (differently than it's been done at the factory.)