The old Pioneer GM 3000 amp is a real classic for some people. It is still used today but it has some old parts that make it hard to install. First of all it has the old din plugs that feature several connections and wires. These can be confusing because they don't mesh well with newer systems. So, you will have to learn which ones go where.
Din Plug Wiring
The wiring on the din plugs usually includes a red, yellow, blue and black wire. In most cases the black wire is not needed or used. The other wires are connected to your amplifier or system. Test them first and use circuits to protect the head units before your power them up.
Since these amps are old it is likely that they have been modified. Most of the cables and plugs on these amps could very easily have been added by the previous owner. Try to use these connections if possible first before you try to sort out which wire is which in the old plugs. The best and easiest way to install is to follow the wiring diagram on the manual and add your own cable connectors. That way you can install it right and protect the rest of your system from damage.