Just checked previous connection post and like a jerk I failed to include amplifier earth connection.....
Now rectified below......
Connection sequence from scratch.....
1,
Remove battery negative terminal
2, Fit new power cable for boot to battery positive terminal, (with fuse fitted)....
3, Go to boot....
4, Connect all speakers to amplifiers
5, Connect all live feeds to amplifiers
6, Connect amplifier remote leads
7, Connect amplifier earths
8,
Double check all connections, (esp power polarity)....
9,
Reconnect main battery earth, (including new earth to match new power)....
10, Start car....
11, Switch on head unit....
12, Check all amps are lit up and running fine, (no protect lights, no blown fuses, no funny smells, no funny noises from speakers)....
13, Switch
off car
14, double check head unit is
OFF....
15, Fit RCA leads into amplifiers.....
16, Double check amplifier gains are at minimum
17, Start car....
18, Switch on head unit....
19, Increase head unit volume & check for background noise.....
20, If alls well, proceed with set up.....
Never disconnect the live battery terminal.....
When connecting the live terminal with earth already connected you will cause a power surge which is quite capable of wiping out a modern cars ECU or other electronic brain units.....
With regards amp protection after RCA fitting.....
OK, when you test the amp with no RCA leads fitted the amp is merely drawing enough current to basically idle.....
As soon as the RCA leads are fitted the amp recieves a signal and so actually draws current to meet its needs.....
If it now trips into 'protect' then this would indicate;
1, Voltage drop----Normally caused by a 'point of high resistance'.....
2, Amplifier fault----Very rare occurence....
3, Wiring issue....
Things to check;
1, All connections are tight....
2, Gain controls are actually fully anti-clockwise, (minimum).....
Things that could be an issue;
1, Remember the interesting earth connection, where did you eventually opt for???
2, Main power feed fuse, is it an 'AGU' type fuse, if it is try another, these notoriously fail and cause resistance induced voltage drop, (along with bags of heat).....
My money would be on your 'AMPLIFIER EARTH'......