by hardwiring it means connect the wires directly without use of harnesses, plugs, connectors etc.
1) fast way
connect the wires from your head unit to the wires already in your car. The colour code for the wires already in your car are at the link below.
2) better way
rewire your car completely - e30s are wired for two channels only, rewire to take advantage of the 4 on your HU. Instructions at link below.
3) the link
any more questions, pm me!