how can i fix my bosch gbh 4 dfe?

i have dismantled the gearbox housing as it had seased with dust in the grease , i have cleaned and freed the clogged cogs , re greased all the internals but i am having difficulty getting the pin on the selector switch to locate into its metal spring type guide , this is stopping the case going back fully and the switch will not select the 3 settings . is there a strateagic way to locate the pin . hope u can help as it is very frustrating ? many thanks.