Every part except for Wheels uses Power in order to function. Each chassis has a certain Power capacity, which depends on the number of Stars that the chassis has. For example, if the Power capacity of your car is 10, you can't install a Chainsaw that requires 11 power onto it.
The only way to increase the Power of your chassis is to fuse a Power Toolbox into it. Power toolboxes can be found in Supply Boxes.