Step 1: Buy head unit, wiring harness, and mounting kit
Step 2: Remove dash surround
Step 3: Remove stock head unit
Step 4: Disconnect Wiring
Step 5: Wire wiring harness to adapter provided with H/U, except red/orange accesory cable.
Step 6: Install mount in dash
Step 7: Pull wires through mount
Step 8: Cut orange amp power cable from the harness that was connected to the stock h/u
Step 9: Wire orange amp power cable with red/orange accesory cables
Step 10: Connect wiring harnesses
Step 11: Install Head Unit
Step 12: Re-install dash surround
Should help, if you have any questions, ask.
1994 Infiniti Q45: Grey metallic exterior, light grey interior, 132k