If you want to execute maintenance for your TGB 303R then it will always be a great idea to have a Workshop Manual to keep just for a guide in case you want to know tips on how to remove a specific part such as the Door and fit it again adequately.
TGB 303R Workshop Manuals can provide specific diagnostic info with regards to your automobile so if something is playing up it will help you work out what the issue is and the way to resolve it. You will be able to analyse if you can do this on your own or in case the job is simply too big you can still book your 303R into a trusted friendly mechanic.
TGB 303R manual