The invention relates to a method, performed by a first control device, for controlling physically connecting a first and a second module to assemble a vehicle, wherein the first control device is comprised in the first module, the method comprising: activating a sensor device in the first module; identifying an area between the two modules by means of the sensor device; transmitting information about the identified area to the second module; continuously determining the position of the second module in relation to the first module, by means of the sensor device, while the second module is moving towards the first module, and transmitting the determined position to the second module; and physically connecting the modules when the second module has reached the first module.