Hello again Rodi...
I was supposed to phone you this morning but was in hospital last night until this morning with a sick little baby.... all ok now..
Anyway I'll phone you today, but in the mean time do have a look at the manual for the amp as followws:
Most amplifiers will have GND (negative '-') terminal connected to the amplifiers chassis. So the actual metal chassis of the amplifier itself is the same and the GND pin electrically.
If your amplifier is mounted on some metal part of the car, that is, the chassis of the amplifier is effectively connected to the negative terminal of the battery already.. I'm guessing when you connected the +12 lead to the GND pin on the amp all you did was blow an in-line fuse on the heavy gauge cable. Check and see if the fuse on the heavy cable near the car-battery terminal is ok.
But, then you did mention that there was a LED on the amplifier. According to the manual on page 20 if the LED is green then all is good, but if it's RED then you have a fault. There are a number of fuses you need to replace to be 100% sure.. .See Page 20 Items 12. Replace the fuses even if you think they look ok to be on the safe side.
After that, if you're sure the fuses are ok.... one other thing you will need to check is the remote power line (REM).. this toggles from 0V to 12V depending on whether your stereo is on or off... do you have access to a multimeter to measure power?
One quick check is to disconnect the REM cable and short the REM terminal to the +12V terminal.
Excuse the post as I'm typing on a phone! I'll try and call you later today to discuss further...