its very easy. all you need to do is:
1. run live wire from battery to boot, adding inline fuse at battery end of cable.
2. run remote wire from back of head unit to boot
3. run RCA's from back of head unit to boot (do with remote wire)
4. fix earth in boot to good earth on chassis (any big bolt will do nicely)
run the wires under the interior plastics so they are hidden, you dont need to take anything off, just slot the wires behind.
to get into the engine bay for the main live wire i suggest you route it through the rubber gromit that is already used for other wires. finding this can be a bit tricky, and i dont know where it is on your car since i havent tried it on a GP before, but it will be there and it is the best thing to use for this job. some people drill a hole through the firewall into the engine bay, while thats an option its not sometihng i would advise anyone to try.