square plug centre pic: boost control solenoid
engine harness plugs into engine bay harness one end and plugs into body harness other end.
engine bay harness provides power feeds from relays for ECU, fuel pump, IGN,.
most other wires are for speed sensor ,A/C, ABS etc
get a diag or you will be totally lost
i did electronic engineering and i needed the diag.
its not as hard as it looks but you need as much info as you can find
if you get stuck PM me
dazza