I read the FAQ here on cprogramming.com just to make sure I was doing it right and there was no problem in theory.
However it seems that my compiler (command-line Borland) doesn't compile the external sources. I know this is a C-board and the FAQ entry is in C++ however the same problem arouses when compiled as a C++ app, just different error messages.
The errors I get are "unresolved external" for each function/variable that is used from the foreign files.
And I haven't forgotten to put prototypes, structures and the likes in the headers either. From what I understand the compiler includes a header file like src1.h, and then automatically looks for a source that accompanies it like src1.c, but it just won't work. I tried compiling it with MSVC++ too and that didn't work either.