afternmarket radio problems

John pretty much hit the nail on the head with his post.  I'll be honest and I'm not trying to sound mean but if you didn't know

1) not to cut the factory harness off,

2) not to cut multiple live wires at the same time

3) where the remote turn on wire goes

I would suggest having the system installed at a professional installation shop because the things that are issues are extremely basic and would be considered things that someone should know without a doubt before even attempting any form of car electronics installation. 

May I ask how the power wire was run from the battery to the amplifier.  Was it taken through a grommet on the firewall or a hole drilled into the firewall with a grommet inserted or through the fenderwell/door? 

as far as the speaker not playing, transfer RCA's between channels and see if the issue follows the RCA.  If that is the case it may be a bad RCA cable which is not at all uncommon especially if cheap RCA cable were purchased.  It could also be a dead channel on the preamp outputs of your headunit.  If the problem continues to stay at channel 4 then it is most likely an issue with the 4channel amplifier

Also do not purchase a test light they are highly recommended against because they are not safe for all circuitry.  They pull a lot of current through the testing device when compared to a DMM and can harm sensitive data wires connected to vehicle electronics such as the PCM,BCM and such.  A DMM is safe for all ciruitry and is stupid easy to use.

Who is John the Hammer?