When you're cutting on or near a vehicle while wrapping, having the right knife is absolutely crucial. You want a light-duty utility knife. Avoid your average X-Acto knives and heavy-duty knives. Look for a multi-cartridge knife with a rounded, plastic front so that you can push the knife against the body of a vehicle and keep a steady hand without fear of scratching anything. You can expect to use at least 2-3 blades during a single vehicle install, so a multi-cartridge knife comes in very handy and can save you valuable install time.
For general signage cuts and windows, a single-cartridge metal blade will do.
Click the links under "Shop + More" to shop the tools shown in this video, along with other light-duty knife options.