libobd is packaged separate from carman. In scratchbox, you can download the source using "apt-get source libobd". This downloads and unpacks the source to libobd-0.7. The file that has to be modified is obd-chan-elm.c in libobd-0.7/src. You probably should also make a note to libobd-0.7/src/debian/changelog to distinguish your modified version.