Problem
I usually load a helping TeX file that contains my customizations, e.g., usual packages, corporate standard colors, and macros for abbreviations. I want my helping file to also load another external file that is in the same folder. I certainly want to avoid giving the explicit path to the second external file every time.
Example
Let's say the helping file is cosmetix.tex, which loads abbreviations.tex. abbreviations.tex is in the same folder as cosmetix.tex and both are in a folder called common:
% cosmetix.tex
% ... some customization
% load abbreviations
\input{abbreviations.tex}
My main file is tester.tex:
% tester.tex
\documentclass{article}
% load cosmetix
\input{common/cosmetix.tex}
\begin{document}
Dummy text.
\end{document}
This leads to the following error: LaTeX error: File `abbreviations.tex' not found.
Question
How can I use nested \inputs with files in different folders without (i) converting my code to a package and (ii) using absolute paths?
Context
Although this question is pretty generic, it is also a follow up to my previous question: Reproducing an official letterhead. I have reproduced my institute's letterhead, which in turn needs to load the logo. I load the letterhead using the \input mechanism. I know this is not the most elegant way, but until I get time to convert this to a package (or read scrlttr2 documentation to convert my letterhead to an option file), this is a solution that reasonably works for me.
As I mentioned above, I can't use absolute paths because I'm using multiple computers with different operating systems, and therefore, the root of the directory tree is different for each computer.

../common/(with as many../needed to get to a common point) assuming that you keep all the files at the same hierarchy depth. Otherwise you could define a\Pathwhich has the full path and use\input{\Path/common/<file>}. – Peter Grill Jun 17 '12 at 01:57~/texmf/tex/latex/commons. See http://tex.stackexchange.com/a/1167/9626 by @JosephWright – Ali Mehrizi Jun 17 '12 at 05:22import, which is written exactly to serve this purpose. – Ali Mehrizi Jun 18 '12 at 05:43