Electrical connectors can be found in various places in the car. A cable lug can have different diameters. They are often arranged by colour, with red having the smallest diameter of 0.5-1.5 mm, followed by blue with 1.5-4 mm, and finally yellow with the largest diameter of 4-6 mm. You simply place an electrical spade connector over the wires and then crimp the sleeve using crimping pliers.
At Winparts, you will find sets of electrical connectors with different sizes and ends. If you do not have crimping pliers, you can order them separately, or you can order a special set of pliers for cable lugs.