You can find TomTom’s GitHub repository at https://github.com/tomtom-international. Source code and modifications are published here.
This site
Source code and modifications for some of the older products are published on this site.TomTom HOME
TomTom HOME is a PC application that manages TomTom GPS devices (Portable Navigation Devices) connected to the PC. It uses Mozilla XULRunner as platform which is being shipped with TomTom HOME. TomTom made some small changes to Mozilla's source code, mostly bug fixes and branding, which are published here.
The source code changes are available under the MPL, GPL and LGPL.
Note for clarification: The actual GUI source code (JavaScript, XUL, locales etc.) is not open source, neither is the "HomeServices" DLL (implemented as XPCOM module written in C++) used to access the device, neither is the HOME2 server. The source below contains only the XULRunner platform and changes to it.