To unlock all visual upgrades for single and multiplayer mode, simply complete Underground mode.

Effect - Code

All Vinyl - Complete Underground mode
All Decals - Complete Underground mode
All Paint Colors - Complete Underground mode
All Accessories - Complete Underground mode
All Rims - Complete Underground mode
All Body Parts - Complete Underground mode
All NOS Packages - Complete Underground mode
All Neons - Complete Underground mode
All Exhaust Packages - Complete Underground mode
All Driveterrain Packages - Complete Underground mode