Impossible problem?

When you say the parts tested ok on the bench, I'm sure you're referring to each one separtely.  Why not try to hook everything up together outside the car.  Maybe then it may be easier to see the issue and also easier to switch a unit out to test with.

I would test that his HU works with Another amp and sub, and that other HU you had mentioned is able to work with his amp and sub, just to eliminate it further,  these are methods I take when fixing computers or my own car audio problems.

Other then that, good luck.

i had a similar problem which might help.  I have a dvc rockford fosgate and would notice that sometimes it lost power.  turned out to be a sort of cut rca which would work when it was jiggled.  that in turn would either cut half the signal going to the sub or give it it's full potential.  Now that i have my whole system out of my car i also noticed that it had a gash in the middle of the run, probably from running it across something sharp.  sound's to me like this might be it..could also be the rca's aren't making contact at the amp or head unit ( loose fit).

I found out that a bad connection to either right or left channel will seriously lower the sound level of a speaker which is bridged from an amp into dvc woofer.

egghh....just something to think about...don't know what kind of woofer he's running

RCA problems was my first inclination too.  Hmm.  Could be that the amp has bad input pre-amplifiers, or it was damaged by the sh*tty original install.  The HU could be shutting down due to a low impedence load on its line-outs.  Try swapping in another amp to test this.

how many subs are hooked up? are they svc or dvc? when everything works, how long does it play for?  i had a problem with a svc woofer before, it would play when it wanted to. hit a bump and it would come back on. when it stopped playing, if i pushed in on the cone, it would play again. after using my ohmeter, found out it was the tinsel leads between the terminals of the woofer and the cone itself. re-soldered to no avail, it eventually stopped working altogether.

i would also try metering the ac signal rca output from the deck @ the deck. next, meter the same signal @ the rca before the amp. try also continuity test of the rcas.

you mentioned that the amp was rockford. does it have a bass knob? if so, check the connection. rf amps will lose 3db without it hooked up.

do the highs work? 4x6's and 6x9's. most decks shut down the internal amp if a single speaker is shorted to ground or shorted period. if so, that may hinder your signal output to the sub amp. there is only one amp, right? no external x-overs in there? it would help better if there were more specifics about the equipment involved.

 those are just my thoughts...............good luck!!!

p.s. just curious as to the solution of your ordeal, please let us know what you find.